Lidar is an efficient tool for remote monitoring, but the effective range is often limited by signal-to-noise ratio (SNR). The reason is that noises or fluctuations always strongly affect the measured results. So the weak signal detection is a basic and important problem in the lidar systems. Through the power spectral estimation, we find that digital filters are not suitable for processing lidar signal buried in noise. We present a new method of the lidar signal acquisition based on discrete wavelet transform for the improvement of SNR to increase the effective range of lidar measurements. Performance of the method is investigated by detecting the simulating and real signals in white noise. The results of Butterworth filter, which is a kind of finite impulse response filter, are also demonstrated for comparison. The experiment results show that the approach is superior to the traditional methods.