The lasing characteristics are investigated for an InAlGaAs/InP wavelength-scale square cavity laser with a height of 800 nm and a side length of 2.2 μm. The laser is coated by aluminum/silica on the bottom and sidewalls and bonded on a silicon wafer. Single-mode lasing at 1556.5 nm with a sidemode suppression ratio of 21 dB is observed under optical pulse pumping for the square cavity laser at room temperature. The lasing mode profile captured by an infrared camera and the spectrum agree well with the simulated results of the three-dimensional finite-difference time-domain technique. In addition, a spontaneous emission factor of 1.5 × 10 -2 is measured by fitting the output power versus input pump power curve with the dynamical results of rate equations.