The invention provides a blocking device and method for an onboard collision avoidance system. The blocking device mainly comprises an FPGA chip, an amplifying circuit, a high-pass filtering circuit, a voltage reducing chip and an isolating chip. The blocking method comprises the following steps: outputting an A/C mode blocking signal after an A/C mode or full-calling mode P3 triggering signal output by a collision avoidance system decoder is received; and if a full-calling mode P4 triggering signal output by a collision avoidance system decoder is received within a certain time range, stopping the continuous outputting of the A/C mode blocking signal, and otherwise, continuously outputting the A/C mode blocking signal. According the blocking device and the blocking method, the blocking lead output by the onboard collision avoidance system can be regulated according to the requirements, and a lag value of an output blocking signal can be regulated, so that the problem of blocking with other same-frequency band avionics subsystems such as navigation and radar alarms after the signal passes through blocking comprehensive crosslinking equipment of an avionics system is flexibly solved, the radio frequency interference is reduced, and the mounting adaptability of the equipment is improved.