There are many parameters in agriculture that change over time in a sigmoid pattern. In the current study, the double logistic function was used to describe and simulate dry matter (DM) variation of winter rapeseed plant and to explain the growth rate under water stress. Irrigation treatments were full irrigation at all growth stages, water stress during the vegetative stage in early spring, water stress at flowering and podding stages, water stress at grain filling stage and rain-fed treatment with supplemental irrigation at time of planting. A high value for the goodness of fit (0.996) and low value for normalized root mean square error (0.085) showed that the double logistic function can describe and simulate DM variation of rapeseed accurately. DM predicted by the double logistic equation based on growing degree day was slightly closer to the measured DM compared with the DM predicted by the double logistic equation based on days after planting. Results showed that growth rate before the winter cold period was lower than that after this period. There were two maximum growth rates for winter rapeseed: the first occurred before cold period and another after.