The invention discloses an underwater displacement sensor for a load box. The sensor includes multiple displacement protection sleeve pipes installed on the load box, and underwater magnetic displacement sensors placed in the displacement protection sleeve pipes, wherein cables are employed to be connected to the underwater magnetic displacement sensors, each of the underwater magnetic displacement sensors comprises a magnetic rod and a stainless steel tube protective sleeve, the magnetic rod is inserted into the stainless steel tube protective sleeve, and a magnet ring is disposed on the periphery of the top of the stainless steel tube protective sleeve. The lower half of the s stainless steel tube protective sleeve is a counterweight block. The head of inserting the magnetic displacementsensor into the stainless steel tube protective sleeve is provided with a retaining ring. The stainless steel tube protective sleeve is mounted within each of the displacement sleeve pipes. The magnetic displacement sensors of the invention have the characteristics of high precision and high reliability of underwater work, and solve the problem that traditional measurement of displacement is thata reinforcing steel bar is connected to the bottom of the load box and is transmitted to the ground in the displacement sleeve pipes, which has the defects of low accuracy of measurement and inconvenient measurement.