In order to measure the width of stiffener in service, fast, contactless and non-destructive, a laser ultrasonic C-scan measurement method is proposed in this work. First, the ultrasonic wave field induced by a pulsed laser excited at a 2 mm-thick 304 stainless steel plate is studied by modelling and theoretical calculation. Then, the ultrasonic field distribution in the 304 stainless steel plate is obtained by laser ultrasonic experiments, and the results are compared with theoretical calculation. Finally, the laser ultrasonic C-scan is used to measure the width of the stiffeners, and the images are processed by the wavelet denoising and image averaging filtering methods. Experimental results show that the amplitude of the reflected transverse wave excited by the pulsed laser on the 304 stainless steel plate is higher than that of the longitudinal wave when the receiving point is 4 mm away from the exciting point. Using the reflected transverse wave amplitude as the characteristic signal, the laser ultrasonic C-scan measurement is performed on the stiffeners on the 304 stainless steel plate, and the absolute error between the width of stiffener obtained from the image and the actual width is less than 0.05 mm, which meets the error requirements of engineering inspection.