Based on the Poisson probability model of Gm-APD, in the long dead-time case, a new approach to improve the detection performances of Gm-APD lidar was proposed. The formulas of detection performances(detection probability, false alarm probability, and range error) of multiple pulses detection Gm-APD lidar were obtained and analyzed. By using an attenuator to control the intensity of signal and noise, the detection and false alarm probabilities, and range error of multiple pulses detection Gm-APD lidar were all improved. With the change of transmissivity of attenuator, the detection probability has a maximum, and the range error has a minimum. While the target is at the distance of 1.5 km, with the increase of the pulse number and the controlled transmissivity of attenuator between 0.33 and 0.20, the detection probability can be doubled with the sharp decline in false alarm probability, and the range error can be decreased from meters to centimeters.