Abstract: The basic principles of spacecraft relative positioning using X-ray source are presented in this paper. This method can break through the restriction of X-ray pulsar absolute positioning which must select steady periodical pulsars. Time differences are simulated using cross-correlation periodic pulse signal and aperiodic changing signal. Results can be derived from the fact that the change amplitude of signals has significant influence on the precision of time difference measurement. The influences of range and ephemeris error on position precision are analyzed. Relative positioning precisions are calculated by simultaneously observing three to six pulsars which parameters are taken from the pulsar database.