It is essential to analyze the residual stresses of thick coatings since they seriously affect coatings' performance during their service. It is also important to understand the mechanisms by which the stresses arise, to predict and control the stresses for improving coating properties. Because the Stoney formula is commonly used to relate stress to curvature for thin coatings, a new calculation formula was developed to estimate the residual stresses of thick coatings that represent a comprehensive stress state of the coated specimen. Based on the deduced formula and accurate curvature measurements, the residual stresses of detonation gun (D-Gun) sprayed WC-Co coatings with different thickness were obtained. With increasing the coating thickness, the residual stress changed gradually from the tensile nature to a compressive nature. Meanwhile, the coating was in an approximately stress-free state at the thickness of around 365 m. The analysis results emphasized the significance of peening stress in controlling the final stress state of the coated specimen, due to the high spraying velocity and kinetic energy during the D-Gun spraying process. Finally, the effects of residual stress on the mechanical properties of the coating were understood, namely, the compressive stress could significantly improve the coating properties, whereas the tensile stress impaired the coating properties.