Abstract: The monolithic scintillation crystal detector has the ability to position the three-dimension interaction location of the gamma ray in the detector depending on accurate acquisition of scintillation light distribution. Depth estimation of light field can be solved by a lens array in light field technology. A new detector model was designed in the paper which introduced a lens array for positioning the interaction in the monolithic detector. The scintillation light is focused by each lens unit in the lens array. Photons in some discrete pixels of the photon counter array where the scintillation light is focus on are detected to form an image of multiple focus points instead of the scintillation light distribution detection based on continuous crystal. The position of the scintillation light source in the crystal can be reconstructed from the focus image through optical path inversion. The detector model consisting of a monolithic crystal, a lens array and a multi-pixel photon counter was established in the paper. The monolithic scintillator was a lutetium-yttrium silicate (LYSO) scintillator with dimensions 48 mm×48 mm×45 mm. The lens was hemispherical with a radius of 15 mm supposed to be uniformly distributed in 3×3 array and the material was as same as the crystal. The crystal and the lens array were considered as a whole in the simulation. The multi-pixel photon counter was set at 44.4 mm from the crystal surface. The pixel size of the photon counter array was 3 mm and the quantum efficiency was 30%. The axis of symmetry of the three parts was same. The reconstruction algorithm was based on optical path inversion from multiple focus points for locating the 3D position of gamma-ray interaction. The intersection points of the inverted optical paths and the set z plane with different depths in the crystal was calculated in the reconstruction process. When the intersection points converged towards the same point in the z₀ plane, the depth of interaction was z₀ and the intersection point in the z₀ plane was the interaction point. The reconstruction results were analyzed to evaluate the performance of the monolithic scintillation crystal detector based on the lens array. The x/y resolution is better than 1.54 mm and the z resolution is better than 3.13 mm. The two interaction points when coincidence occurs are reconstruced in the condition that the energy is the same. The detector is feasible for the positioning of two interaction points by the reconstruction results of three groups of selected positions. The monolithic scintillation detector based on a lens array has both good positioning ability and position resolution. The detector is suitable for the position reconstruction of multiple interaction points.