The invention, which relates to the technical field of nuclear radiation detecting, provides a synchronous setting apparatus and method of a radon precipitating rate measuring instrument. The synchronous setting apparatus comprises a filling block connected to the inner wall of a radon collection room of a radon precipitating rate measuring instrument detachably and a rapid radon precipitating rate setting unit connected in series to an air inlet pipe or an air return pipe. A radon precipitating rate value measured by the rapid radon precipitating rate setting unit is used as a standard for setting of the radon precipitating rate measuring instrument. To be specific, the rapid radon precipitating rate setting unit carries out equivalent processing on a changing radon concentration by usinga difference method; and on the basis of a corresponding scale factor value and alpha particle counting data collected by a detector during the measuring process, a radon concentration value of a next measuring cycle is calculated by combining a radon concentration value of a previous measuring cycle; The radon precipitating rate setting speed is fast; the time of radon precipitating rate measuring instrument setting is reduced substantially; the deviation caused by too long time during the setting process is reduced; and the setting value accuracy is enhanced.