Based on the physics fact, an extra equation has been introduced to specify the wavelength of the two-segment abeorptive bistable semiconductor lasers(TABLs). With appropriate modifications, conventional rate equations have been used to study the wavelength characteristics of the TABL. Calculations lead to a wavelength loop accompanying the hysteresis process, indicating a wavelength jump towards the longer wavelength at "switch-on" (sudden increase of output power) and a reverse process at "switch-off" (sudden decrease of output power) as shown in Fig.1 and Fig.2.