Abstract: The readings of ex-core detectors contain the information of in-core power distribution,and the relationship between ex-core detector readings and in-core power distribution is the detector space response function.The key prerequisite of using ex-core detector readings to reconstruct the in-core power distribution is to calculate the space response function accurately.The space response function of one pressured water reactor is calculated via Monte Carlo adjoint transport method in this paper by using the MCNP code.The numerical result shows that there are some difference from that obtained via S_N adjoint transport on two-dimensional geometry,and indicates the necessity to calculate the ex-core detector space response function on three-dimensional geometry.Monte Carlo adjoint transport method is convenient to calculate the three-dimensional spatial response function.