Proceeding from the principle of physical quantity measurement with a sensor, the design idea of catching the detected signal by the high-accuracy modulation transmitter and its composition are described by analyzing characteristics of the sensor output signals. The design principle of the key circuits in the transmitter is analyzed in detail. The successful application of the capacitance transmitter in the long range liquid level transmission system of the rocket engine test-stand shows that the new modulation transmitter has the characteristics of high accuracy, generalization and high reliability.