A motion vector determining method includes a first matched distortion value between a forward prediction block and a backward prediction block of a current block is calculated. A matched distortion value between a first prediction block and a fifth prediction block, a matched distortion value between a second prediction block and a sixth prediction block, a matched distortion value between a third prediction block and a seventh prediction block, and a matched distortion value between a fourth prediction block and an eighth prediction block are calculated. A target forward prediction block and a target backward prediction block are determined based on a value relationship between the matched distortion values. A target motion vector of the current block is determined based on the target forward prediction block and the target backward prediction block.