1. A motion prediction method, comprising: (a) obtaining a plurality of motion vector predictors to be included in a candidate set for motion prediction of a current unit of a current frame, wherein the plurality of motion vector predictors comprises at least a spatial motion vector predictor of an adjacent block of the current unit, and at least a temporal motion vector predictor from a collocated unit of the current unit in a previous coded frame; (b) determining whether the current frame is a non-reference frame which is not referred to by other frames for motion prediction; (c) when the current frame is not the non-reference frame, adjusting the candidate set by removing the temporal motion vector predictor corresponding to the previously coded frame from the candidate set to avoid parsing error propagation; (d) when the motion vector predictors are equal to each other, adding at least one backup motion vector predictors until a different backup motion vector predictor is obtained: (e) selecting a final motion vector predictor from the adjusted candidate set by applying rate-distortion optimization; and (f) predicting a motion vector of the current unit according to the final motion vector predictor of the adjusted candidate set.