Aiming at deep space optical pulse position modulation (PPM) communications, based on the principle of expectation-maximization (EM) timing error estimation, a new solution of optical PPM slot synchronization based on serial concatenated pulse position modulation (SCPPM) code is studied. Due to that the expected value of EM estimation can not be got by the standard SCPPM decoding method, a method to convert the soft information output from SCPPM decoder to the expected value of each PPM slot is proposed. Firstly, the hard decision of output from the SCPPM decoder is made, and the accumulator state through accumulation module is got. Then, a mapping method according to the former accumulator state is chosen, and the probability and expectation of each PPM slot is obtained. Finally, EM algorithm is used to predict the clock offset and then to obtain the best sampling point. According to the simulation results, for the asynchronous sampling signal with two times PPM slot frequency, the proposed method can effectively achieve the PPM slot synchronization under the condition of the clock error among －0.5~0.5 PPM slots.