We present a method for complex spectrum analysis of periodic optical signal with multi-level modulation format. The information of the complex spectrum can be extracted with ultra-low sampling rate in the proposed method, and the temporal information reconstructed by inverse Fourier transform can be used to monitor the performance of the optical signal. Besides, the proposed method is transparent to symbol rate and modulation format. The spectral amplitudes and phases are measured accurately in the simulation based on the proposed method for a 28 GBaud Quadrature-Phase-ShiftKeying (QPSK) signal with a 400-bit pattern and a 28 GBaud 16-Quadrature-Amplitude-Modulation(16QAM) signal with a 560-bit pattern. Moreover, we analyze the performance of the reconstructed signal under different linewidth and optical signal-to-noise ratio (OSNR) by simulation. The Q-factor of the reconstructed 28-GBaud QPSK signal reduces when the linewidth of laser increases and the OSNR of signal decreases.