The expandable power adaptor contains a first casing member, a second casing member, a circuit assembly, and at least a metallic connector. The first casing member has a first coupling element set. The second casing member has a second coupling element set. The first and second casing members are joined to form an enclosed accommodation space. The circuit assembly and the metallic connectors electrically connected to the circuit assembly are configured in the accommodation space. To assembly, the first lower piece and the second lower piece are joined to form a complete lower piece. Then the circuit assembly and the metallic connectors are configured on the complete lower piece, and the first and second upper pieces are vertically joined to the first and second lower pieces. Additional casing members can be integrated between the first and second casing members so as to achieve the desired number of outlets.