This paper was focused on deriving the B nomogram of L-moments method,which was used to estimate the sampling errors in frequency analysis.The design floods are always added its mean square deviation to ensure the projects safety.An approximate formula of B based on Monte Carlo experiments,named B nomogram,is often used to estimate the mean square deviation.The L-moments method for flood frequency analysis is very efficient,which has some applications,but the B nomogram for L-moments method isn't available now.Based on Monte Carlo experiments,the B nomogram of L-moments method has been developed and validated.