Prefabricated steel structures are widely used in construction projects due to their advantages such as light weight, high strength, and convenient construction. However, their fire resistance and anti-corrosion performance are insufficient in complex service environments, and they need to rely on multiple protective coatings (such as fireproof layers and anti-corrosion layers) to ensure long-term durability. The precise measurement of coating thickness is a key link in quality control. The existing ultrasonic testing technology is confronted with problems such as echo aliasing at multi-layer interfaces and noise interference, resulting in insufficient measurement accuracy. To this end, this paper proposes a coating thickness detection method based on the combination of high-frequency ultrasonic pulse reflection method and sparse decomposition matching pursuit (MP) algorithm, and improves the detection accuracy through signal separation and reconstruction. Firstly, a finite element model of the protective coating for prefabricated steel structures was established based on COMSOL to simulate the ultrasonic detection process of water immersion, analyze the propagation characteristics of ultrasonic waves in multi-layer media, and reveal the physical mechanism of interface echo. Aiming at the problem of aliasing signal separation, a sparse decomposition MP algorithm that optimizes the over-complete atomic dictionary of Chirp is proposed. Signal denoising and feature reconstruction are achieved by iterative matching of the optimal atoms, and the separation effects of the traditional wavelet transform modulus maximum method are compared. The simulation results show that under the same signal-to-noise ratio condition, the root mean square error of the reconstructed signal by the MP algorithm is significantly lower than that of the wavelet transform method. The relative errors in the thickness detection of its fireproof layer and anti-corrosion layer are also better than those of the wavelet transform method. To verify the practicability of the method, a 20 MHz water-immersed probe was used to test the anti-corrosion layer and fireproof coating test blocks, and the true thickness was calibrated in combination with the metallographic method. The echo of the superimposed interface was successfully separated through the MP algorithm. The relative errors of the thickness of the fireproof layer and the anti-corrosion layer were calculated to be -4.64% and -4.65% respectively, which were significantly better than -7.15% and -7.28% of the wavelet transform. Therefore, the method proposed in this paper has better practical detection application scenarios.