The present disclosure provides a method for decoding a video signal including a current block based on an affine motion prediction mode (affine mode, AF mode), the method including: checking whether the AF mode is applied to the current block, the AF mode representing a motion prediction mode using an affine motion model; checking whether an AF4 mode is used when the AF mode is applied to the current block, the AF4 mode representing a mode in which a motion vector is predicted using four parameters constituting the affine motion model; generating a motion vector predictor using the four parameters when the AF4 mode is used and generating a motion vector predictor using six parameters constituting the affine motion model when the AF4 mode is not used; and obtaining a motion vector of the current block based on the motion vector predictor.