Abstract: To investigate the shear behavior of reinforced concrete (RC) beams strengthened with an embedded inclined (45°) U-shaped carbon fiber reinforced polymer (CFRP) grid and polymer cement mortar (PCM) composite system, four-point static loading tests and finite element analysis were carried out. The effects of the number of CFRP grid layers, the number of strips, and the layout pattern on the mechanical behavior of the strengthened beams were preliminarily compared. By comparing different FRP effective strain models, the shear contribution of the CFRP grid was evaluated, and a preliminary empirical correction relationship for the effective strain of U-shaped CFRP grid strips was established based on the test back-calculation results. The test results showed that the embedded inclined U-shaped CFRP grid-PCM strengthening system increased the shear cracking load of RC beams by 16.67%–25.00% and the ultimate load by 23.54%–31.02%, and delayed crack propagation. Under the test conditions of this study, increasing the number of CFRP grid layers or strips led to a certain increase in the bearing capacity of the specimens. The ultimate load of the specimen with spaced strips was higher than that of the specimen with continuous strips, but its CFRP grid strain distribution was less uniform. The numerical results could reasonably reflect the crack development process observed in the tests. The established preliminary empirical correction relationship for effective strain could reduce the calculation deviation of existing models for the shear contribution of the CFRP grid within the test parameter range of this study.