For the increasingly severe parking problem, a method of parking lot space detection based on a modified convolutional neural network was proposed. Firstly, based on the characteristic that a parking lot only needs to be denoted by two states, a concept of Mini Convolutional Neural Network (MCNN) was proposed by improving the traditional CNN. Secondly, the number of network parameters was decreased to reduce the training and recognition time, a local response normalization layer was added to the network to enhance brightness correction, and the small convolution kernel was utilized to get more details of the image. Finally, the video frame was manually masked and cut into separate parking lots by edge detection. Then the trained MCNN was used for parking lot recognition. Experimental results show that the proposed method can improve the recognition rate by 3-8 percentage points compared with the traditional machine learning methods, and the network parameters of MCNN is only 1/1000 of the conventionally used convolutional model. In several different environments discussed in this paper, the recognition rate maintains above 92%. The experimental result shows that the MCNN can be transplanted to a low-configuration camera to achieve automatic parking space detection.