Ultralow-loss laser coatings have important applications in precision measurement fields such as gravitational wave detection， optical atomic clocks， and optical cavity ring-down spectroscopy. The cavity length stability and total optical loss of the laser resonator determine the sensitivity and signal-to-noise ratio of the measurement system. With the development of thin-film materials， fabrication processes and detection techniques， significant progress has been achieved in thin-film optical loss and thermal noise research. As regards optical loss， the absorption of the thin film can be controlled at the sub-ppm level， and the scattering of the thin film has become the main contributing factor of optical loss. This paper focuses on defect-induced scattering and interface scattering and presents the research ideas and achievements of thin-film scattering control. The interface scattering of thin films is reduced through optical factor design and interface power spectral density control. The theoretical analysis model of nodule defect-induced scattering is established， and the physical mechanism and the control technology of the defect-induced scattering are proposed. With regard to thermal noise research， this paper primarily introduces the physical mechanism of thin-film mechanical loss， presents the mechanical loss reduction of reflective coating through thin-film material optimization， and the corresponding improvement in the characterization method of mechanical loss.