A novel frequency stabilization method for the seed laser of the pulse optical parametric oscillator (OPO) in differential absorption lidar (DIAL) is proposed in this paper. The operating principle and the experimental setups are introduced in detail, and the accuracy of the frequency stabilization method and influence factors are also analyzed. The frequency of the seed laser is locked to an absorption line of water vapor (935.6849 nm) in the experiment, and the measured standard deviation of the frequency jitter is less than 8 MHz. The frequency of the seeded OPO signal is recorded thereafter. The standard deviation less than 28.7 MHz with the same frequency according to the seed laser is achieved, which is well applicable to DIAL.