Abstract: Abnormal liquid level fluctuation in the thin slab continuous casting mold easily disturbs the stability of the meniscus, causes mold powder entrapment and uneven solidification of the shell, which is a major inducement for inclusion defects of cast slabs. This study, based on the process parameters of a thin slab mold in a domestic steel plant, established a water model with a similarity ratio of 0.5 to the industrial prototype. A wave height meter and a three-dimensional particle velocimetry system were used to collect liquid level fluctuations and surface velocities, and wavelet transform was combined to perform quantitative analysis of the liquid level fluctuations. The results show that the main frequencies of fluctuations in the mold are concentrated in the range of 0-1.09 Hz. With increasing casting speed, the frequency composition of fluctuations exhibits a trend of first increasing and then decreasing, while the amplitude gradually increases. As the SEN (submerged entry nozzle) depth increases from 130 mm to 170 mm, the amplitude gradually decreases, and the wavelet energy at each location migrates to lower frequency bands. Specifically, the proportion of low-frequency (0-0.135 Hz) energy near the SEN increases from approximately 0.35 to 0.75. Furthermore, the total wavelet energy and the average wave height show a consistent trend, with the wide 1/4 region being the main area for fluctuations. The amplitude is positively correlated with the surface velocity, but there is no obvious correlation with the frequency. This study reveals the time-frequency evolution law of fluctuations in thin slab mold, providing a reference basis for stable control of the mold level fluctuation and dynamic diagnosis of operating conditions.