Based on Monte Carlo simulation of neutron penumbral imaging，the linear space invariance is researched. In the field of view，the points broadening linearly depends on the axial distance on the object plane，but the integrated intensity keeps linear space invariance. Using this result，the alignment precision is analyzed. The axial error of the targets center on the object plane should be controlled within 50μm. The point source should be less than 300μm away from the object plane. The aperture rotation should be within 0.1mrad.