The state preparation flaw which is generated by the pulse fluctuation in the signal applied for the phase modulator in BB84 protocol lead to the reduction of the security final key generation rate. We proposed DPM as a robust modulator to the inaccuracy in the applied voltage in BB84 protocol by using optical phase as the encoding technique. We experimentally proved that the key generation rate and the fidelity of the state generated by DPM are better than DDM.