This paper proposed and realized a novel all optical pressure sensor. It is achieved by using fiber coupling laser diode (LD) to pump the Nd∶YAG microchip. The pressure was imposed indirectly on the radial direction of the crystal by a pressure transferring device. As a result of the stress birefringence the microchip laser′s longitudinal mode was split into two with different frequency. The frequency splitting signal is coupled into fiber, detected by a photodiode and than changed as a detectable beat frequency signal. There is a direct proportional relation between the frequency difference and the pressure imposed on the microchip, so pressure can be obtained. Moreover, the effect of the modes on the measurement error was finished. A result of 1.31 MHz/kPa was achieved in the experiment, so it can be used in the high sensitivity measurement and has a broad application future.