The pressureless screen filter installed in front of the pump can effectively solve the problems of high energy consumption and large pressure loss of the conventional filter which is installed behind the pump. In order to evaluate the filtration effect more accurately and comprehensively, the commonly used pre-pump pressureless screen filter in Xinjiang was taken as the research object, and laboratory and in situ experiments were carried out simutaneously. In the experiments, the concept and calculation method of the effective filtration efficiency were proposed. The experiment results show that the larger the sand content before filtration, the greater the average filtration efficiency of the total sand. However, the total sand filtration efficiency at a certain moment does not necessarily increase with the increase of the sand content before filtration, reflecting that the filtration efficiency is not only related to the sand content, but also related to the mesh diameter, particle size and particle group size distribution. Before and after filtration, the change of particle group for fine sand is not obvious, whereas that of the coarse sand changes significantly. Therefore, the screen of the filter is only effective for the filtration of sand particles with a particle size larger than the mesh diameter (0.180 mm in this study). According to the proposed concept of effective filtration efficiency, the average effective filtration efficiency of coarse and fine sand was calculated to be 95.5% and 98.1%, respectively. As the particle size increases, the effective filtration efficiency of the particle group also increases gradually, indicating that the filtration efficiency of the screen for the coarse particle group is significantly greater than that for the fine particle group, which further proves the rationality of the proposed effective filtration efficiency in this study. The research results can provide a calculation basis for a more accurate and comprehensive evaluation of the filtration effect of the screen filter, and supplement the insufficiency of the calculation method of filtration efficiency in the existing specifications.