The invention discloses a TDR (time domain reflectometry) based float switch type water level sensor and a TDR based float switch type water level measurement method. Two core wires are arranged from a water pump to the ground in an axial direction of a water pump lift pipe, a plurality of float switches are uniformly distributed along the two core wires at intervals, the float switches and the two core wires are bound on the water pump lift pipe, and a TDR measuring instrument is connected with the float switches through the two core wires; on-off of the two core wires is controlled through vertical floating of floating balls in the float switches, so that short dot position data are acquired and transmitted to the TDR measuring instrument; before measurement, the sensor is placed in a measured well and is calibrated, the position of a corresponding short dot in TDR sampling data when each floating ball is submerged is recorded, the TDR measuring instrument repeatedly samples the short dot during measuring, if the dot position variation is smaller than a threshold value, an average value is taken and compared with a calibration value obtained before measurement, and an actual distance between the water level and a wellhead is determined. The TDR based float switch type water level sensor has high capacity of resisting converter interference, can realize high-precision deep-water measurement, is relatively convenient to maintain and replace and is particularly suitable for being used in the well where a variable frequency pump works.