The pulse spreading is one of the dominant factors that influence the ranging performance of the airborne laser scanner. Firstly, the dependence of the backscattered pulse width on the flying height and the scanning angle was analyzed, and then the attenuation of the backscattered power due to pulse spreading was investigated. The influence of the pulse spreading on the system’s Signal-to-noise Ratio (SNR) and the ranging accuracy was studied. The maximum flying height of aircraft was analyzed based on the scanning angle and the platform flying height and the system’s parameters. The simulations indicate that the width of the back-scattered laser pulse increases considerably from the centre to the edge of the air strip. The received power and SNR decrease sharply with the increase of the scanning angle. The ranging accuracy is variable, and decreases sharply from the centre to the edge of the air strip.