An wind turbine blade mould side adjustment device is provided, characterized in that the device is provided at a flange of the mould, and the side adjustment device comprises an upper part and a lower base, wherein the lower base is fixed on the mould frame and the upper part is capable of sliding over the lower base in a direction substantially parallel to the blade width direction. Such an improved side shape adjustment device for wind turbine blade moulds makes adjustment of the mould width easier and more efficient, and meanwhile allows the mould to readily expand in the blade longitudinal direction.