A method of measuring the lowest energy in the detection system was proposed. The system in which the power of subpulse is equable and the system in which the power of subpulse is inequable were studied by the proposed method, the results show that the energy efficiency of input intensity are 41.3% and 28.8%, respectively. The threshold energy of the charge-coupled device which used to detect the interference fringe is measured of 56.7 mW by using Mach-Zehnder interferometers. The energy two kinds of detecting systems need are 0.86 W and 1.575 W respectively calculated by the measured energy efficiency and threshold.