The invention provides a gusset plate for a solar panel rack and a fixing structure for the solar panel rack, which can restrain stress concentration on support pillars and are light. The fixing structure for the solar panel rack (1) is characterized in that a pair of gusset plates (50, 50) is used for clamping butt joint parts of a rafter support part (20) and the support pillars (40) and studs (B2) are used for connecting the two gusset plates (50, 50) from the two sides of the gusset plates (50, 50). The fixing structure is characterized in that the gusset plate (50) on one side is composed of a T-type gusset plate (55) comprising a flat plate part (56) extending along the rafter support part (20) and the support pillars (40) and a protruding plate part (57) protruding from the flat plate part (56) in a right-angled manner; the gusset plate (50) on the other side is composed of an I-type gusset plate (51) comprising a flat plate part extending along the rafter support part (20) and the support pillars (40), wherein the flat plate part (56) and the protruding plate part (57) are provided with a plurality of bolt holes (58, 59).