Abstract: For reducing the computation complexity of pulse period ambiguity resolution for X-ray Pulsars NAVigation (XPNAV) and improving its convenience in engineering implementation, the Optimization Model for 4 X-ray pulsars localization is presented based on the orthogonality principle. A new relation formula of period number of the Time Difference Of Arrival (TDOA) of pulses is presented, whose factors are decided by the characteristics of X-ray pulsars and whose constancy is decided by TDOA phase residue. Theoretical analysis and experimental simulation show that the new method can reduce the complexity of computation obviously. Meanwhile the relation formula of the new ambiguity resolution has constants, which can be solidified into hardware or be stored into memory unit, so that the new ambiguity resolution is easy to implement in engineering.