A weight training equipment with an ultrasonic sensor assembly including a transmitter unit, a receiver unit, a case and the support. The case is rotatably supported upon the support to expose the transmitter and the receiver upwardly. A freely rotatable pin unit is selectively inserted into the hole of the corresponding weight block of the weight training equipment to have its reflection surface downwardly face the sensor assembly. A distance between the pin unit and the sensor assembly during exercise can be measure by ultrasound transmitted from the transmitter unit toward the pin unit and further reflected by the reflection surface and received by the receiver unit.