The invention discloses an airborne collision avoidance system (ACAS) tester and a testing method. The tester comprises a signal processing module, a testing ACAS host radio frequency module, a testing S-mode responder radio frequency module and an interface extension, wherein the signal processing module comprises a microprocessor, an FPGA (Field Programmable Gate Array), a signal driving and isolating circuit, a configuration chip and a voltage conversion circuit. The signal processing module accomplishes coding of a transmitting-receiving host inquiry signal of an ACAS, analyzes an S-mode protocol, judges an inquiry format, codes according to an inquiry mode, generates a responding signal, generates an A / C / S-mode inquiry signal for an S-mode responder of the ACAS, codes the responding signal, analyzes the S-mode protocol, and obtains the height and identity data (an S-mode address) of the S-mode responder, and requirements on functions such as blank pipe response, air traffic alarm, ADS-B OUT (Automatic Dependent Surveillance Broadcast Out) of the ACAS tester are met.