Abstract: The fabrication and assembly errors of the drift tube linac (DTL) may cause a large discrepancy on the distribution of the accelerating field. The tuner was typically used to compensate the deviation. Meanwhile, a series of post-couplers were introduced to increase the stability of the accelerating field by properly adjusting the gap between post-coupler and drift tube. In this paper, based on RF resonant cavity theory and bead pull technology, a high-precision measurement system was established. Combining the simulation with experiment, the low power resonant frequency tuning of CSNS DTL tank was successfully performed. The relative deviation between real field distribution and design result is less than 2%, and the resonant frequency and stability reach the tuning objective.