A precision conversion methodology with truncated normal distribution theory assumption oriented to maximum-error specification was brought forward, and it could be taken as a reference frame for the precision conversion between maximum-error specification and other precision measurement specifications, so that the precision class of according true value measurement systems could be determined in advance. The method assumes that the conformity probability of the observation sequence is subjected to logarithmic truncated normal distribution; based on the aimed confidence level for maximum-error specification and the given sample size of target sequence, the calculation formulation of upper truncated limit, lower truncated limit, mean and standard deviation of the truncated normal distribution were proved and derived, thus the precision conversion relationships between maximum-error specification and other precision measurement specifications, such as 1σ, were turned out; through referring to the corresponding theories on precision instrument fields, the determination methodology for precision class of true value measurement systems under maximum-error specification was given. The application on related example cases proved the feasibility of the proposed method.