The hybrid waveform combined Linear Frequency Modulated and Pseudo-Random Binary phase Code(LFM-PRBC) is a new type of low probability intercept signal, which is widely adopted in radars and small detectors. It is significant to estimate the parameters of the hybrid waveform. This paper develops the Modified Cramer-Rao Lower Bound(MCRLB) for the modulation parameters of this signal with Gaussian noise. The modulation parameters considered are signal amplitude，carrier frequency, modulation slope and code width, and the nuisance parameters are codes and initial phase. Conclusions indicate that MCRLB is independent with nuisance parameters，and MCRLB in logarithmic form is linear with the Signal to Noise Ratio(SNR) when the vectors of estimated parameters are determined.