Abstract: Strong-motion data are essential to earthquake engineering studies. Recorders for strong-motion usually work in the triggering mode. Various types of such recorders e.g., the PDR-1, PDR-2; PDR-100, PDR-200; GQ-Ⅱ, Ⅲ and the QZ-2A are now in use, and seismic displacement, velocity, or acceleration signals have been used as trigger signals. But the chief difficulty encountered is how to make the triggering action immune to various kinds of interference so as to keep missing triggers and false triggers to a minimum. This paper describes an interference-immune trigger circuit. It is now in use in the GQ-Ⅱ and QZ-2A strong-motion recorders which have been tested in the field. Simulation interference tests show that the rates of both missing triggers and false triggers are below one percent.