The performance of ToF (Time of Flight) measurement for ultrasonic wave can be obtained only by numeric calculation in traditional narrow band model. To this problem, a method based on Laguerre model is proposed for CRLB (Cramer-Rao Lower Bound) calculation. The characteristics of Lagueree function and its time derivative were studied, and the time derivative inner matrix was derived, which was applied in Fisher matrix, then the CRLB can be calculated fast and accurately. The simulation on hybrid exponential model polluted by Gaussian white noise testified the well coherence between the variance of ToF and the theoretic CRLB obtained with Laguerre model. The variance of ToF obtained by real data has great difference with the theoretic bound for the impact of narrow band noise and reverberant interference.