The invention discloses a slot type solar reflector detection device and a detection method thereof. The slot type solar reflector detection device comprises a first working support and a second working support. A first movable guide rail assembly and a second movable guide rail assembly are arranged at the lower ends of the first working support and the lower end of the second working support respectively. A first guide rail is connected between the upper end of the first working support and the upper end of the second working support through a support height adjusting device. The first guiderail is connected with a photoelectric detector stepping device and two light source stepping devices. The light source stepping devices are located on the two sides of the photoelectric detector stepping device respectively. A light source arranged below the light source stepping devices are connected with the light source stepping devices. The photoelectric detector stepping device is connectedwith a connecting rod. One end of the connecting rod is connected with a spherical photoelectric detector. Compared with the prior art, the device is high in environmental adaptability and can be adapted to the test requirements of slot type solar reflectors different in height, width and length. Meanwhile, the test data of the device are accurate.