Abstract: The oscillator method is one of the methods for measuring small reactivity. According to the point-reactor kinetics equation, the expression of zero power reactor (ZPR) transfer function was deduced. Based on the Fourier analysis of the triangular wave input, the expression of oscillation reactivity was gotten. Validating research was made on ZPR, including oscillation experiments of control rod and a sample. Comparing with the triangular wave technique research of the reactivity measurement and the period method, the result is agreement.