An apparatus and a method for frequency locking are provided. The apparatus includes a phase-locked loop (PLL), a local clock generator, a data buffer unit and a control unit. The PLL locks the phase and the frequency of a radio frequency signal to generate a recovery clock signal and received data. The data buffer unit writes the received data into an elastic buffer of the data buffer unit according to the frequency of the recovery clock signal, and reads the received data from the elastic buffer according to the frequency of a local clock signal generated by the local clock generator. The control unit obtains a write-in address and a read-out address in the elastic buffer, and sends a control signal to the local clock generator for adjusting the frequency of the local clock signal according to relationship between the write-in address and the read-out address.