Abstract: In order to optimize the micro-lysimeters (MLS) and improve its measurement accuracy, influential elements of accuracy was studied by the field testing without changing soil in 15 days. Result showed that the accuracy of all test treatments was high. The highest point was up to 97.40%. Whether the inner cylinder was drilled or not did not affect obviously the measurement accuracy. The accuracy of the drilled outside cylinder was higher than the undrilled ones, and the drilled holes were better to be equally distributed. Further more, the holes number was less than 36 when the diameter was 3 mm. The sealed outside cylinder could lower the measurement accuracy. The thinner material of sealed inside cylinder was and the more ventilative function was, the higher accuracy could be achieved. Moreover, when the space between inside and outside cylinder was smaller, the measurement accuracy also became higher. Dew studying results indicated that dew at night could markedly lower measurement accuracy of MLS. When the condensed quantum was above soil evaporation, MLS could not accurately measure out soil evaporation. Meanwhile, the measurement accuracy varied from time to time. The soil water content in the first 3 days was more, but the accuracy of MLS was lower than the later days. If there was available water supply, it was practicable without changing the MLS soil in 15 days.