A time measurement system for an airborne LIDAR based on constant-fraction discrimination is designed by using FPGA. The principle and characteristics of time discrimination, especially of constant-fraction discrimination are described in detail. The error of the traditional constant-fraction discrimination is estimated and a simple circuit for constant-fraction discrimination is designed. Finally, a preliminary scheme for implementing time measurement for a LIDAR by using a tapped delay line method is given. Some suggestions on the improvement of the time measurement system in accuracy are made.