This paper develops a depth measurement theory related to defect width for the quantitative laser ultrasonic detection of surface defect depth. A depth measurement formula with a width correction item is established, and a concept of defect size ratio is defined. Defects are divided into three types, namely narrow defects, extremely narrow defects, and wide defects. The applicability of the defect depth measurement method in measuring the three types of defects is discussed, and finite element simulation is adopted for verification. Finally, an experimental platform for laser ultrasonic detection is built to conduct depth detection of aluminum alloy samples with surface defects. The results show that quantitative detection of narrow defect depth can be achieved by introducing the width correction item. The average measurement error is less than 5%, which means that accurate measurement of the defect depth has been achieved.