The strength in heat affected zone of welded aluminum alloy decreases sharply. One of the effective measures is to use CFRP sheets for reinforcement. A shear experiment of 37 double lap joints of aluminum alloy reinforced with CFRP sheets, including specimens with different CFPR bond length, bond width, bond layers, single layer thickness and bond form, was conducted. Failure modes, bond mechanism between CFRP sheets and aluminum alloy, and strain distribution were studied. Besides, effective bond length and ultimate load calculation formulas of the double lap joint were proposed.