In order to overcome the shortcomings of precast composite beams, such as easy cracking in post-cast zone, poor ductility, and easy to damage, engineering cementitious composites (ECC) were used in the precast plate and post-cast area of composite beams. Bending tests were carried out on 11 beam specimens. The flexural properties and effects of the ECC plate thickness, connecting method of reinforcement, and reinforcement ratio on the flexural properties were investigated. Bearing capacity and displacement of its characteristic point were studied. The crack width was measured and the laws of crack development trend and damage evolution were analyzed. The results show that the load, displacement, and ductility of the precast beams with ECC plate and post-cast joints under cracking, yielding, and ultimate condition were higher than those of RC beams. The thicker the ECC plate was, the more significant the effect was. The reinforcement connection methods of steel sleeve and hoop-inserting could effectively transfer the reinforcement stress. The precast beam could effectively control the crack width in bending area, and the damage index was larger when it was damaged. However, it could still bear load when the damage was serious, which is suitable for the key joint of precast structure.