The invention discloses a locking fixture for a screw on an L-shaped mainboard. The locking fixture for the screw on the L-shaped mainboard comprises a bottom plate, insulating support pillars, limiting blocks, a stepped front positioning block, a multi-stepped height adjusting block, a screw template, a template base bar and a side edge type locking device, wherein the bottom plate is provided with a plurality of insulating support pillars and limiting blocks corresponding to the mainboard; the front side edge of the bottom plate is provided with the stepped front positioning block; the back side edge of the bottom plate is provided with the template base bar; the upper edge of the template base bar is provided with the reversible screw template through a hinge; the right side of the bottom plate is provided with the side edge type locking device; the bottom plate is also provided with the multi-stepped height adjusting block; the multi-stepped height adjusting block is positioned on the left side of a push rod of the side edge type locking device. Therefore, the locking fixture is simple in structure and convenient to use, the operation of a worker is intuitive, and a mainboard positioning locking device is arranged to effectively prevent the mainboard from shifting.