A solar thermal collector and an accessory structure of a building are provided. The solar thermal collector includes at least one heat absorbing plate and at least one heat insulating plate. Each of the heat absorbing plate includes at least one first slab and first engaging parts connected with the first slab. Each of the heat insulating plate includes at least one second slab and second engaging parts connected with the second slab. The first engaging parts are respectively engaged with the second engaging parts, and a gap is maintained between the first slab and the second slab to define a heat collecting channel, through which a heat transfer fluid flows between the heat absorbing plate and the heat insulating plate. A heat conductivity of the heat absorbing plate is at least 30 times greater than a heat conductivity of the heat insulating plate.