Abstract: The basic theory of tolerance analysis technology is discussed. The technology precept of circuits tolerance analyses and design based on the EDA software PSpice is given. The methods of Monte Carlo analyses and worst-case analyses are discussed in detail. Three aspects of the application fortolerance analyses, circuits parameter deflection, effecting of temperature and degradation, are studied. An example of Monte Carlo analyses is given, the upper limit value and the lower limit value of system performance reliability are computed. The effecting between system performance and variance of circuit component parameter is analyzed by running multiple simulation. Feasibility of the precept is proved by the analyses of the example.