The invention discloses a total hydrocarbon tester having the on-line attenuation function. The tester comprises a sampling pipe, a pump, a hydrogen flame ionization detector, a compressed air bottle and a mixed gas chamber. An input end of the mixed gas chamber is connected with the sampling pipe, the other input end of the mixed gas chamber is connected with an air pipe, and the air pipe is connected with the compressed air bottle and provided with an air flow control device. The output end o the mixed gas chamber is connected with the pump through a mixed gas flow control device, and the pump is connected with the hydrogen flame ionization detector. Sample gas enters the mixed gas chamber, is mixed with clean air and reaches the hydrogen flame ionization detector under the action of the pump, and the hydrocarbon content of the sample gas is tested. The mixed gas flow control device and the air flow control device cooperate to control the proportion of the sample gas to the clean air, and the on-line attenuation function is achieved. By means of the total hydrocarbon tester, the concentration of hydrocarbon in the sample gas entering the hydrogen flame ionization detector is lowered in a quantitative mode, and shutdown of the tester due to overload or insufficient oxygen in the sample gas or other reasons is avoided.