There is an urgent need for blood oxygen saturation (SpO2) tests when participants are ambulatory, as in daily activity monitoring, sleep monitoring, or even athletes' cardiovascular function tests. In such situations, measuring equipment needs to be wearable. This restricts the processor volume, and the corresponding algorithm should be microprocessor compatible.This article proposes an anti-motion interference blood oxygen saturation algorithm for the microcontroller based on AC and DC analysis, named de-trended FFT.An experiment was conducted to compare the de-trended FFT algorithm with two other algorithms commonly used in the time and frequency domains. In the experiment, participants' oxygen saturation levels were calculated from Photoplethysmography (PPG) signals that were recorded continuously. Meantime, five types of hand motions were conducted, including hand trembling movements, horizontal hand movements, vertical hand movements, finger tapping, and finger bending, with each state lasting 2 minutes.Results show significant performance of de-trended FFT in SpO2 calculation (P < 0.05), in both accuracy and stability.De-trended FFT stands out in both mean deviation and variance by eliminating trending influence when compared with the other two algorithms. The motion interference's influence on SpO2 calculation mainly comes from the AC component, not the DC.