Abstract: Abstract: Studies on how to improve the photosynthesis rate and yield of tomato, which are mostly affected by water and fertilizer, are receiving more and more attention. However, little is known about the coupling effect of irrigation and fertilizer on tomato's photosynthesis rate, yield and water use efficiency (WUE). Our research was to find the best water-fertilizer coupling effects, to reveal the relations between two, and to provide data support for high yield and WUE for tomato production. Randomized block design through quadratic regression orthogonal rotary method with 16 treatments and three replicates was used in this research. The irrigation lower limit was 50% of field capacity, irrigation maximum and fertilizer were divided into 5 levels (60%, 66%, 80%, 94% and 100% of field capacity; N:P2O5:K2O 125:63:125, 200:100:200, 375:187:375, 550:275:550, 625:313:625 kg/hm2, respectively). Each block was 7 m×1.2 m with 25 plants; blocks were separated by 50 cm deep plastic film to prevent the cross penetration of water and fertilizer. Tomato Jinpeng 1 seedlings with 5 leaves were transplanted with double rows of 60 cm row spacing and 30 cm plant spacing in plastic tunnel on March 9, 2013 and uprooted on June 20, 2013. Ten days after transplanting, 3 fertigation devices (MixRite-2054, Israel) were used for irrigation through mulched drip irrigation. Soil moisture was monitored using TDR (TDR 300, USA). When the moisture was lower than the lower limit, drip irrigation was supplied to the corresponding maximum; the drip velocity and time were measured and recorded. Fertilizer of each treatment except P2O5 was equally supplied for 5 times (before transplanting, the 1st, 2nd, 4th and 6th fruit enlarging period); P2O5 was all supplied as base fertilizer before transplanting. The photosynthesis rate was measured with portable photosynthesis systems (Li-6400, LI-COR Biosciences Company, USA) between 9:00-11:00 am on May 30, 2013. The yield and the quantity of water and fertilizer for each treatment were recorded. By building a mathematical model about its photosynthesis and yield, the coupling of water-fertilizer was analyzed and studied. The results suggested that the two models were significant, and the determination coefficient R2 was 0.93 and 0.95, respectively. Photosynthesis rate was more influenced by water than fertilizer, however the yield was opposite. The water-fertilizer coupling and yield were positive related. A significant ascend trend of photosynthesis occurred with the increase of irrigation maximum, and it began to decrease when the irrigation maximum reached a certain amount. Moreover, disregarding the amount of irrigation water, the photosynthesis increased first and then decreased slowly. The yield increased remarkably with an increase in irrigation water and fertilizer of a certain range, but decreased when the quantity was exceeded. Water and fertilizer affected yield and WUE due to their impact on photosynthesis. The yield wasn't linear correlate with photosynthesis. The medium water and high fertilizer were favorable for tomato photosynthesis, but lower in fruit yield compared with medium water and medium fertilizer. It also demonstrated that optimizing proportion of irrigation maximum and fertilizer was favorable to increase the yield and WUE, which provided water and fertilizer use guideline for high yield and highly efficient production of tomato. The most suitable combination of irrigation and fertilizer was as follows: irrigation lower limit was 50% of field capacity, irrigation maximum was 80%-82% of field capacity, and the fertilizer was 313.75-439.75 kg/hm2 of N, 156.55-219.19 kg/hm2 of P2O5 and 313.75-439.75 kg/hm2 K2O. Under this combination, the highest yield and WUE were 124 t/hm2 and 43.2 kg/m3 water, respectively. Verification test was conducted from 5 August, 2013 to 18 November, 2013 using these rates of fertilizer and water at South Horticultural Field of Northwest A&F University showed a similar result. This indicated that the models were reliable and could provide guidance for agricultural production.