In this work, the method, in principle ofthe box complex algorithm was adopted to obtain stress-induced crystallization coefficient C and the strain-optical coefficient $A_{op}$ with the value of 295 and $1.5{ imes}10^{-9}$, respectively, and some parameters $A_1=0.27$, $A_2=5.06$, a= 3.5, b=1.8 relative to the elongational viscosity of poly(trimethylene terephthalate)(PTT) fiber. The vitrification distance as a function of the take-up velocity and mass throughput was also gotten. The effects of spinning conditions on filament temperature, velocity gradient, spinning tension, birefringence and crystallinity, and effect of viscoelasticity on take-up velocity had been discussed.