[en] We, first, demonstrate an optimized structure design and analyze the optical mode properties of 1.3 μm wavelength square microcavity lasers on silicon with InAs/InGaAs quantum-dot active region and mid-point output waveguides. Si-based square microcavities with whispering-gallery-like modes have been proposed for optimizing their quality factors (Q factors). Three-dimensional finite-difference time-domain method is used to numerically analyze the optical mode characteristics. The influences of the side-length of the microcavity, the output waveguide width, and the etching depth on the optical modes of the microcavity are investigated in detail. It indicates that the Q factor increases with increasing etching depth, and decreases rapidly as the waveguide width increases. The results show that with the side length of 18 μm, the waveguide width of 1.0 μm, and the etching depth of 3.5 μm, the Q factor is the highest, and the mode distribution is optimal. The mode wavelength and Q factor are 1305.9 nm and 4694.8, respectively. Advantages of more stable and evenly distributed mode profiles for Si-based square microcavity lasers have been demonstrated, and compared with the Si-based disk microcavity (microdisk) lasers. It promises a potential alternative laser structure for Si-based optoelectronic integration.