Abstract: Abstract: Fine roots are the most important and active part of the root system. Due to the inaccessibility of root systems, special techniques are required to investigate the fine root distribution and root growth parameters. Conventional method to study the root system such as the quadrate monolith method always consume large labors, cause destruction to the root system, can't monitor the root growth in suit and will cause the error of the measurement. While minirhizotron technique provides a nondestructive, in situ method for viewing roots growth images and is one of the best tools available for directly studying root system. In order to meticulously investigate the growth dynamic of cotton fine root under film mulch drip irrigation, a field experiment was conducted in 2014 by using minirhizotron technique to measure the growth of cotton root in situ at the Bazhou Experimental Station in Korla, Xinjiang, China. Reference standard used in this experiment was quadrate monolith method. The minirhizotron and quadrate monolith methods were applied to analysis the growth of cotton root under mulched drip irrigation with fresh water, and the root growth parameters of the two methods were analyzed to establish a regression model. Results showed that, the cotton new root and dying root could be judged by the colour of root from the root images, and the variation parameters of length and diameter of cotton new root and dying root can also be directly estimated from the cotton root growth images collected from one of the minirhizotron tubes at the depth of 10-20 cm. The cotton new root growth turn over at the depth of 10-20 cm was 1.844 mm/d with minirhizntron method, more and more root turned to be dying root as time gone on in the periods of blooming stage to boll opening stage. The parameters of mean diameter, total projection area and total surface area of cotton new root increased with time increasing, while those parameters of dying root decreased with time increasing. The deepest growth of cotton root in soil was observed at 50 cm through the two methods. The total root length density determined by using minirhizotron technique in the blooming stage (08-07), boll stage (08-17 and 08-27), and boll opening stage (09-06) were 5.49, 5.90, 5.59 and 4.49 cm/cm2 respectively. The total root length density determined by quadrate monolith method in the blooming stage (07-16), boll stage (08-17) and boll opening stage (09-06) were 3.29, 5.14 and 5.78 cm/cm3 respectively. The distributions of cotton root length density at different depth of soil were the same observed through the two methods, root length density was increased with the increase of soil depth between 0-30 cm, the maximum root length density was observed at the depth of 20-30 cm, root length density was decreased with the increase of soil depth between 30-50 cm. The results of the root length density among two methods were significant correlation at 0.99 confidence level. The equation of linear regression could be used to estimate the actual root length growth parameter after we acquired the root length growth parameter with minirhizotron method. It further showed that the minirhizotron technique could provid as a reliable, nondestructive and in suit method for directly viewing and studied the growth of cotton fine root change within soil under mulched drip irrigation, and by using the root processing software (WinRHIZOTron MF 2012) we could easily acquire the root parameters after collected the root growth images. For better understanding the meticulous growth dynamic of cotton fine root, steps must be taken to insure good soil/minirhizotron tubes contact without disturbing the soil; increasing the monitoring scope and the frequency of cotton root growth images collection through different periods of cotton growth will be needed in further research.