The invention discloses a support-beam-type MEMS (micro electromechanical systems) fluid thermal conductivity and thermal diffusion coefficient sensor and preparation and testing methods thereof; a heater is arranged at the center of the upper surface of a substrate, a pad is arranged beside the heater on the upper surface of the substrate and is connected to the heater, film of an insulating layer covers the heater, the pad and the substrate, the film of the insulating layer is provided with cavities on two sides of the heater, the bottom of the substrate is provided with a back cavity, and a support beam structure is formed. The heater also acts as a temperature sensor; the heater is of elongated strip structure and is connected to the pad through four leads; by using the back cavity, a fluid under test is used as a substrate of the sensor, thermal conductivity and thermal diffusion coefficient of the fluid under test can be measured directly, and the test process is simplified. The sensor provided herein is suitable for measuring the thermal conductivity and thermal diffusion coefficient of conductive and nonconductive trace liquids.