The invention relates to a transmittance detector for a dyeing wastewater control system. The transmittance detector used for detecting clarity of sewage is composed of a main shell, a detection T-branch pipe, a light emitting component, a light measurement component with signal output, and a drainage housing, wherein the detection T-branch pipe, the light emitting component and the light measurement component with signal output are all installed in the main shell, and the detection T-branch pipe is located between the light emitting component and the light measurement component; a water sample inlet pipe, a first air inlet pipe and a second air inlet pipe are arranged on the top of the main shell in parallel; the detection T-branch pipe comprises an upper-end water inlet pipe, a left water outlet pipe and a right water outlet pipe, a first gap is reserved between the light emitting component and the left water outlet pipe, and a second gap is reserved between the light measurement component and the right water outlet pipe; output ports of the first air inlet pipe and the second air inlet pipe face downwards and are located above the first gap and the second gap respectively. By the adoption of the transmittance detector, the situation that detection accuracy and stability are affected by surface dirt can be prevented, cleaning and maintaining frequency can be reduced, and detection accuracy and stability of the detector are improved.