The image definition criterion is one of basic problems in passive autofocusing technique based on digital image.The traditional Brenner Algorithm has a fast calculation speed,but the accuracy of evaluation result depends on the threshold value,and the Algorithm sensitivity is low. For this issue, an improved Algorithm is proposed. The improved function uses high pass filter and bandpass filter to evaluate the image, and overcomes the limitation of traditional Brenner Algorithm depending on the threshold value.In order to evaluate the improved Algorithm performance, the parameters of unimodality, accuracy, sensitivity, calculating cost are compared and analyzed.Compared with the traditional Brenner Algorithm, the experiments and analysis show that the improved Algorithm can meet unimodality, accuracy, and improve sensitivity and reduce calculating cost.