To solve the problem that low measuring accuracy and bad realtime caused rocket engine working condition analysis difficult. Based on the characteristics of a rocket engine turbopump rotational speed signal and telemetry signals rules this paper proposed a cycle method measuring the rotational speed of liquid rocket engine turbopump based on CPLD. Designed the speed signal cycle extraction, transmission and channel coding method in telemetry system. Using the Quartus II software simulation process of speed cycle measurement. An original products took part in the telemetry system test. The results show that digital engine speed measurement method better than lower frequency method in measurement accuracy and realtime performance. It can solve the problem that rocket engine is immeasurable In the period of Speed change quickly like accelerating period and shutdown periods.