A pulsed time-of-flight laser radar operates by transmitting a short laser pulse to an optical visible target and detecting the reflected pulse by an optical detector. The measured distance is calculated from this flight time. So the accuracy of laser time interval is a basic beacon to measure its system capability. By using two tapped differential delay lines working in a matrix mode with delay locked loop (DLL), to gether with counter, high precision measurement in flight time is realized in an entire digital way with all measurement construction located in a single field programmable gate array (FPGA). The single-shot precision of the time digital converter (TDC) is better than 250 ps, the drifts of TDC units remain within±30 ps over a temperature range -10～+50 ℃. The concrete design of transmit and receive circuits is discussed to improve the accuracy of the TDC. Because of the high integration, low power, light weight and stable performance the prototype reaches the request of the laser altimeter.