To study studied the behavior of the tripod type four level atomic medium driven by three light fields in resonant system, then put it to the unidirectional ring cavity. Through the study we found that the driving field and control field frequency, the cooperation parameters, and the detuning can influence the produce of optical bistability in different degree. The bitable threshold width and adjustable parameters are given it can provide a theoretical basis for experimental control and realization of optical bistabilitv.