The MEMS accelerometer has wide applications in object posture monitoring as a low-cost inertial measurement unit. According to the mathematical model of the three-axis digital MEMS accelerometer, the scale factor, zero bias, and error on the installation of the accelerometer are derived in detail. A simple six-position calibration method based on cuboids is proposed. In comparing the results with those of the three-axis turntable calibration, the six-position calibration method is shown to be simpler, more accurate, and easier for the MCU to achieve, which is suitable for places that lack a three-axis turntable. The six-position simple calibration method for the three-axis digital MEMS accelerometer thus demonstrates good universality.