Background and objective: Femtosecond laser (fs-laser) is a novel tooth preparation tool. After cavities were prepared by fs-laser, their axial-wall taper is closely related to the retention force and success of restorations, while the taper after preparation by fs-laser is not clear yet. This study aimed to explore the value of the taper after fs-laser tooth preparation and the effect of laser fluence, scanning line spacing, and scanning layers on taper. Materials and methods: Twenty cavities with a width of 0.5mm were prepared by an fs-laser both in enamel and dentin to obtain the axial-wall taper (No. PKUSSIRB-201949124). To study the effects of three parameters of fs-laser, five different laser fluence were set as 1.56, 3.13, 4.69, 6.25, and 7.81 J/cm² of five cavities. Scanning line spacing was set as 6, 12, 25, 30, 40, and 50 lm of six cavities and the scanning layers were 5, 10, 25, 50, 75, 100, 200, 300, and 400, respectively, of nine cavities. Laser pulses irradiated perpendicular to the tooth and the scanning path was controlled via a vibratory mirror. The taper was measured with a laser confocal microscope. Curves of the taper versus different parameters were drawn. Results: There was a significant difference in taper between enamel and dentin. Overall, the taper decreased gradually with the increase of laser fluence. The optimal taper obtained was 20.53°- 0.28° in dentin and 38.71°- 0.46° in enamel when the corresponding fluence was 6.25 J/cm². In comparison, the scanning line spacing has little effect on the taper. The scanning layers significantly affect the taper, which decreased dramatically as scanning layers increased within 75 layers and does not change considerably as it was beyond 100 layers. Conclusions: Reasonable selection of laser fluence, scanning line spacing, and the number of scanning layers can optimize the cavity taper by fs-laser. [ABSTRACT FROM AUTHOR]