An effcient entanglement distribution protocol was proposed using a qudit (4-dimensional space) as a separable mediating ancilla. Two qubits were used to describe a qudit in order to simplify the description. Firstly, both of the two ancilla qubits and the two distant qubits (a and b) were formed into a four-qubit unlockable bound entangled state, called the Smolin state. Then, a projective measurement in the Bell basis was performed on the two ancilla qubits and the measurement results was delivered to a and b. Based on the measurement results, the qubits a and b can be converted into a standard singlet state locally. At any stage of the protocol, the two ancilla qubits was always separable from the subsystem (a and b). This scheme provides an effective way to distribute the maximally entangled states by separable states, and presents a concise formalism to describe and understand this physical process.