In this work, we study the bistability of an active nonlinear microring resonator and design a flip-flop based on the active microring resonator. In the presence of nonlinear and linear loss, we use Er-doped gain medium in the microring to obtain gain to compensate for the loss of the resonator. Both analytical and numerical methods are used to solve the propagation in the microring with double couplers, and we obtain the hysteresis loops of the microring. The results obtained by the two methods reveal that, in the presence of nonlinearity in microring resonators, the system exhibits bistability, and the gain in the microring leads to a decrease of the bias power when the active microring is taken as a bistable switcher. Basing on the bistability of the microring, we realize a set-reset flip-flop by adding a positive or negative feedback onto the bias. We also find that the duration of the set and reset pulses must exceed the field buildup time of the microring if we want to achieve the switching of the bias signal. In our design, the duration time is about 2 ps.