The invention discloses a bridge crack detection device and a detection method. The bridge crack automatic detection device includes a control device, an installation trolley, a telescopic rod mechanism and a detection device. The control device includes a data processing device, a display, a walking controller, a camera A shutter frequency detection unit, a telescopic rod adjustment device, a telescopic rod adjustment controller, a first distance detection unit and a second distance detection unit, a lighting brightness detection unit, a brightness adjustment module and a lighting brightness controller; the detection method includes steps: 1. Preparatory work before detection; 2. Image shooting and uploading; 3. Image interception; 4. Image processing of bridge cracks; 5. Image calibration of calibration objects; The invention has the advantages of simple structure, reasonable design and convenient use and operation, can realize accurate and rapid detection of bridge crack size, and improves the working efficiency of bridge crack detection and the reliability of detection data.