A novel combined modulation detector with a combined format of Pseudo-Random-Binary-Phase Code (PRBPC) and Linear Frequency Modulation (LFM) is proposed. Its anti-reconnaissance capability is analyzed from three aspects. The reconnaissance distance for the reconnaissance equipment to scout the combined modulation signal is derived; a model of parameter acquisition probability is established with the help of the window function theory; the expression for the probability of intercept parameters is obtained; the distance cut-off feature of the combined modulation signal is achieved by the half-width of the main lobe of the signal autocorrelation function. The combined modulation waveform is compared to the single LFM signal and the single PRBPC signal during the analysis. The computed results of the three aspects for typical parameters show that the combined modulation detector has a quite large reconnaissance distance due to its short-range working environment, the parameter acquisition of the combined modulation signal is very low, and its distance cut-off feature is very incisive. It is concluded that the combined modulation detector enjoys a strong anti-reconnaissance performance.