Semiconductor microring lasers (SMLs) have received increasing attention in the areas of all-optical logic and random storage owing to their unique characteristics for wavelength conversion, tunability and optical bistability. Based on the analysis of relationship between backscattering coupling and SML operation regimes (bidirectional continuous-wave, bidirectional alternate oscillations, unidirectional bistability), the structure parameters and process of microring resonator are optimized, and an InP-based low threshold SML with unique unidirectional bistability is fabricated. The experimental results indicate that the laser with a threshold current of 56 mA has a lasing wavelength of 1569.65 nm. Once the driving current exceeded the threshold, the SML entered into the unidirectional bistability without any bidirectional regimes. Thus the driving current and power consumption of bistability operation are reduced greatly, and the device reported here is very suitable to the optical random storage memory unites.