The invention provides a basic computing unit and a computing method of a convolutional neural network. The basic computing unit includes a controller, an addition tree, an input buffer, several computing units, and an output buffer; the computing unit includes a block random access memory, Several convolution operation units, internal adders, activation pooling units. Based on the control of the controller, the input buffer loads the image data of the corresponding number of lines to each calculation unit, and the block random access memory sends the effective number of lines and the initial number of lines to each convolution operation unit so that it can obtain the image data of the corresponding number of lines; The convolution operation unit processes the image data and sends it to the addition tree through the internal adder; the addition tree processes the image data sent by each internal adder and sends it to an activation pooling unit; the activation pooling unit processes the image data and sends it to the output terminal cache. The solution can realize the algorithm based on hardware, so that the completion time of the algorithm can be controlled.