The invention discloses an online blowing thermal gas mass flowmeter which comprises a thermal mass flowmeter gauge outfit and a sensor probe which is connected with the gauge outfit through a connecting rod, wherein one side of the sensor probe is provided with a blowing pipe, a blowing port is formed in the blowing pipe, and is aligned to the sensor probe, and a one-way air inlet valve is arranged on the blowing pipe. The online blowing thermal gas mass flowmeter has the advantages that when the one-way air inlet valve is filled with nitrogen, the nitrogen is conveyed to the sensor probe by the blowing pipe, so the nitrogen is directly blown to the sensor probe to blow dirty materials which are adhered to the sensor probe, so the measurement precision is ensured.