From the literature of the threshold type decision pulse position keying system, it is found that the theoretical formula given by the literature cannot describe the Bit Error Rate(BER) accurately. Especially under low signal to noise ratio, the BER calculated by the theoretical formula is bigger than 0.5, which is in inconformity with communication theory. The problems of the literature in deducing the formula are analyzed and a new theoretical formula is given for the BER of the threshold type decision pulse position keying system. Simulation results show that the new theoretical formula can describe the BER accurately, and the BER calculated by the theoretical formula is below 0.5 under low signal to noise ratio, which is in conformity with communication theory. Based on the new theoretical formula, the energy efficiency of the threshold type decision pulse position keying system is analyzed and compared with that of Binary Phase Shift Keying(BPSK) system. The results show that the threshold type decision pulse position keying system is energy efficient and even better than BPSK system in some situations.