The invention discloses an underwater penetration sensor in a water quality detector. The underwater penetration sensor in the water quality detector is characterized in that the water quality detector is provided with a water level sensor and an underwater sensor; the electric block of the water quality detector is provided with a rope drum winding number sensor. When the water quality detector is used for detecting water quality, the water level sensor transmits signals after the detector arrives at the water level; the rope drum winding number sensor on the electric block starts to accountthe underground penetration, so that the water quality detector can detect a plurality of water quality indexes according to the appointed water depth accurately; when the detector arrives the water bottom, the underground sensor transmits a signal, and the electric block stops lowering the water quality detector. The underwater penetration sensor has the characteristics of high precision, good resolution, reliable working, simple operation, automatic control and the like; and the underwater penetration sensor is an advanced detection auxiliary apparatus.