The invention provides an outdoor air pipe connection device for preventing rainwater backward flowing, and a mounting method of the outdoor air pipe connection device. Through an oblique connecting pipe, a horizontal cross pipe and a rainwater reciprocating preventing bent pipe arranged at the wall passing position, when rainwater leaks into an air pipe through a felling seam, accumulated water does not flow backwards into the indoor space along the air pipe due to the fact that the rainwater backward-flowing reciprocating preventing bent pipe has the height difference, after the rainwater leaks into the air pipe, the water is accumulated at the low position of the rainwater backward-flowing reciprocating preventing bent pipe, and during overhauling, the accumulated water in the air pipecan be drained only by demounting the horizontal cross pipe. After the rainwater leaks into the outdoor air pipe through the air pipe felling seam, rainwater backward flowing can be effectively prevented, and the further influence on the indoor air quality and tail end equipment is avoided. In addition, by using the oblique connecting pipe to be connected with a vertical pipe, the rainwater backward-flowing reciprocating preventing bent pipe is arranged at the wall passing position, the horizontal cross pipe is connected with the outdoor air pipe or equipment, through the combination, the airresistance can be reduced to the maximum extent, and air flow smoothness is ensured.