To evaluate the cost risk and the schedule risk of the spaceflight project,  the schedule-cost (S-C) correlation coefficient is directly appointed according to the experts' experience usually.  This paper deduces SDMCU (the schedule distribution model considering the effect of the cost uncertainty),  and then proposes the approximate formula to estimate the ln (S)-C correlation coefficient based on the models of SDMCU and CDMSU (the cost distribution model considering the effect of the schedule uncertainty).  Furthermore,  an approximate relationship expression of the S-C and the ln (S)-C correlation coefficients is put forward according to general facts and the Taylor expansion,  and advanced by means of mass numerical validation is the general rule of obtaining the estimation value of the schedule-cost correlation coefficient based on the historical data.