To investigate the shear performance of composite shear connectors under biaxial load, a finite element model was established and verified by experimental tests. The failure modes of composite shear connectors under biaxial load and unidirectional push-out load were compared, and the relationship between biaxial load and shear strength ratio was also analyzed. The effects of concrete strength and through reinforcement diameter on the shear strength under biaxial load were clarified. The shear mechanism of composite shear connector was revealed. A shear strength model of composite shear connector under biaxial load was established by multivariable regression analysis. The results show that lateral pression can effectively prevent the transverse expansion of concrete, and a longitudinal main crack and a few transverse microcracks appears in the concrete slab. Under the action of unidirectional push-out load, the composite shear connector fails due to the penetration of splitting cracks in the concrete slab. Lateral pression increases the shear strength of composite shear connectors, but the biaxial load effect is small when the concrete strength is high and the diameter of through reinforcement is large (shear strength ratio is 1.05). The shear strength of composite shear connector significantly increases with the increase of concrete strength and through reinforcement diameter under biaxial loading.