In this thesis, we present the phase-intensity conversion transmission method for the first time. We analyze phase-intensity conversion theory, laser diode noise, and EDFA noise. By experiment, we investigate the impact of the parameters in phase-intensity conversion such as light source linewidth, phase modulation index, dispersion magnitude, and EDFA on the system performance. We successfully demonstrated simultaneous transmission of one AM-VSB channel through intensity modulation and one QAM channel through phase modulation over 10 km single mode fiber using LiNbO3 external modulator. The system performance with CNR of 57~61 dB for the AM-VSB channel and BER of 6.8´10-9 for the QAM channel can be achieved. The demonstration and the experimental results confirm the feasibility of phase-intensity conversion techniques. Potentially, the phase-intensity conversion transmission method could be applied on optical encryption and decryption systems.