This paper investigates the relationship between climatic signals and noises,and the problem of upper limit of accuracy of long-term forecasting monthly mean temperature,and concerns the evaluation by three methods of the variance of climatic signals and noises from the daily temperature series of 25 stations in the study area.Results show that,with the absolut eerror smaller by a factor of 0.68 than mean square deviation as the criterion of correct prediction,the upper limit would be within 66%～72%,with little difference between the winter and summer prediction.