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In order to understand the correlations between the laser shock processing and materials’ response, an as annealed plain carbon steel 45 (equivalent to AISI 1045) was investigated by means of a Nd:glass laser setup with 1054 nm wavelength and 20 ns short pulse. The microstructure evolution was analyzed with optical microscope, scanning electron microscope, etc. The experimental results indicate that the strengthening effects can be obtained under the used confinement conditions and the ultra-high strain rate of laser shock. After laser shocking, the cementite in pearlite crashes the subject to their orientations, and the pro-eutectoid ferrite produces a great number of dislocation etched pits and lens-like twins. The hardness of pearlite and pro-eutectoid ferrite increases by 70 and 55 Hv, respectively, and the residual compressive stress of the steel reaches 425 MPa on the shocked spot.