Direct path interference, because of the small path loss, has a much greater interference intensity than the reflected echo of the target, which directly affects the detection performance of the passive bistatic radar target. Intelligent reflect surface (IRS) has the ability to control the propagation of electromagnetic waves. For this reason, an IRS is placed near the radar receiver. By optimizing the beamforming of the channel receiver and the reflected phase of the passive IRS, the direct path interference power entering the receiver can be constrained to a certain range, and the target echo power can be maximized, thus improving the target detection performance. The joint optimization problem is transformed into two homogeneous and non-homogeneous quadratically constrained quadratic programming problems by using alternating optimization method. The optimum solution for beamforming of receiver and reflective phase of IRS is obtained by using semi-definite relaxation method and Gaussian randomization method. Computer simulation analysis shows that the detection performance of passive bistatic radar under direct path interference can be effectively improved by using the IRS.