Benefiting from low peak-to-average power ratio (PAPR), constant envelope OFDM (CE-OFDM) is a promising technique in coherent optical communication. However, the spectrum efficiency of CE-OFDM is only half of conventional coherent optical OFDM, which restricts its application. We proposed a novel quasi-constant envelope OFDM (Quasi-CE-OFDM) scheme with in-phase/quadrature (I/Q) modulator. Furthermore, a phase demodulation scheme based on Taylor series expansion is performed to reduce calculation complexity and avoid phase ambiguity. Simulation results show that the proposed scheme could double the spectrum efficiency effectively without losing the advantage of high nonlinearity tolerance.