Based on the nonlinear characteristics of mutually injection,the theoretical model of mutually injected system using the vertical cavity surface emitting laser(VCSEL) has been established.The analytical expression of mutually injection locking range is deduced by this model.Also the types of synchronization and the influence of injection delay are studied.The results indicate that under the ideal condition without parameter mismatch and noise,the complete synchronization can be achieved,and the injection delay has much effects on the state of VCSEL.Under the real condition with parameter mismatch and noise,the complete synchronization will be replaced by the delay synchronization,and the synchronization error changes in step shape as the injection delay changes.Simulation results also validate the expression of locking range.