A detection and diagnosis has been performed on the bad effect of the central air conditioning system of a theater，and the site test on the actual operating conditions of the plate heat exchanger has also been proceeded. In this paper， based on the correction of the measured heat transfer obtained by actual measurement，the specified logarithmic mean temperature difference heat transfer rate and the specified flow heat transfer has been obtained. By using the specified heat transfer rate as a comparison and regarded as a basis of judging of the performance of plate heat exchanger. Finally， the corresponding recommendations about the actual operation of plate heat exchanger and the steps of the performance diagnostic has been put forward in this article.