The invention discloses a collection/expansion type mixing module used for a gas transmission system of an atomic fluorescence spectrometer. A mixed gas input area and a mixed gas output area are communicated through the collection/expansion type mixing module, and the collection/expansion type mixing module has multiple corners. A first radial passage and a second radial passage at two ends of the module are communicated through an axial passage and form a corner; the cross section of the axial passage takes the shape of a round table; and a contraction port and an expansion port are arranged at two ends respectively. In the mixed gas input area, uneven mixed gas of to-be-measured metal gaseous hydride, gas (hydrogen), water vapor, argon and the like advances in zigzag, and is continuously crashed and mixed fully and uniformly through compression and expansion corners and the like, so as to obtain stable laminar flow combustion flames, as a result, the stability of the testing result is improved. The mixing passage is provided with the corners, and the axial passage is provided with the contraction port and the expansion port; in coordination with bosses on the inner wall and through multiple times of experiments, the range of the optimal included angle between the contraction port and the horizontal level can be obtained, as well as the optimal specific value of the height of the radial passages to the length of the axial passage.