Flexible optical transmitters can define the modulation format of a signal according to the requirements of different application scenarios, further optimizing the transmission performance of an optical signal. A flexible high-order quadrature amplitude modulation (QAM) optical transmitter scheme based on a dual-parallel Mach-Zehnder modulator (MZM) and a single MZM cascade is proposed in this paper. The principle and method of generating and adaptive-switching circular 16QAM, four-amplitude-four-phase 16QAM, circular 32QAM, and four-amplitude-eight-phase 32QAM optical signals are theoretically analyzed. In the virtual path identifier (VPI) simulation environment, the generation and transmission performance of the above four signals under 5 Gbaud and 10 Gbaud modulation rates are verified. The results show that the transmitter supports free switching between 8/16/32QAM signals by configuring the drive signal (binary or quaternary) and the direct-current bias voltage of the transmitter. Furthermore, the four generated signals have a good signal-to-noise ratio after transmission through a specific length of a single-mode fiber.