The utility model discloses a transmission type detector for waveguide signal power detection, which comprises a lower cavity with a small hole and a waveguide, a substrate and a standard SMA (Shape Memory Alloy) connector, the substrate is stuck in the channel of the lower cavity, one end of the substrate is extended into the small hole of the lower cavity, and the output of a detector circuit on the substrate is connected with the probe of the standard SMA connector. The circuit form of the substrate can be of a microstrip line structure or a coplanar waveguide structure. Because the structure of the transmission type detector provided by the utility model is not related with the wavelength of the waveguide in the lower cavity, the power detection of broadband can be carried out, the mechanical position of the device does not need to be changed when the fluctuation of in-band microwave power is tested, and consequently, the affection of device abrasion on testing is reduced, and the stability of testing is enhanced. Meanwhile, the structure of each part and the relative positional relation between the parts are regular, so the parts are easy to process and assemble, and moreover, because the loss of the main waveguide is little, the changes of the incident wave and the reflected wave in the main waveguide can be simultaneously reflected.