In order to study the effect of laser power density on principal stresses and their directions by laser shock processing (LSP). 2024 aluminum alloy samples are processed with five different laser power densities. Residual stresses of three directions are measured by X-ray diffraction (XRD), and the principal stresses and their directions are calculated. Experimental results show that effect of laser power density on the amplitude of principal stresses and the spread of its direction, stress intensity and maximum shear stress by LSP. The variance of maximum principal stress and its direction of 2024 aluminum alloy are 1987 and 13905 respectively, with the laser power density of 2.8 GW/cm2. Residual stress is of uniform distribution, and the angles of principal stress directions are scattered. But the average of maximum principal stress of 2024 aluminum alloy is -158 MPa. The average of maximum principal stress of 2024 aluminum alloy is -239 MPa with the laser power density of 2.1 GW/cm2. The variance of maximum principal stress and its directions of 2024 aluminum alloy is 5471. Residual stress is less uniformity.