The hybrid modulation quantum key distribution (QKD) via single-photon two -qubit states scheme is analyzed,and an improvement scheme is proposed.The input-output mod el of polarization is established.And the influence of the interferometers on the pol arization coding is analyzed.Quantum bit error rate (QBER) of polarization coding caused by the phas ic error of fast and slow axes and the alignment error between the basis vectors of polarization and polar ization axis is simulated.Furthermore,the stability conditions of polarization a re derived on this basis.The QBER of polarization coding can be influenced deepl y by the fiber M-Z interferometers.The theoretic analysis reveals that the jitt er of polarization state is not only derived from the birefringency,but also der ived from coordinate system difference between incident light and fast and slow axes in fiber interferometer.Some necessary conditoins for the QKD via single-p hoton two-qubit states are given.The simulation show that when ρ and θ are both below 0.2rad,the polarizatio n QBER is less than 0.015.When ρ and θ are both below 0.4rad,the polarization QBER is less than 0.06.The pola rization QBER of 2%－5% in two-qubit QKD scheme could be accepted as far as the safety upper limit of QBER (11%) is concerned.