In order to improve observation precision of the meteorological visibility under non-ideal conditions, the equivalent visual threshold and general Koschmieder′s law are put forward and demonstrated based on the atmospheric optical transmission theory. Under the non-ideal condition, the observation visibility formulas of relative error and the correction coefficient are established. The weather visibility relative error and the correction coefficient of the non-black targets are analyzed under the conditions of non-parallel level of the sky background. Results show that the correction coefficient of visibility observation is influenced by target reflectivity, background reflectivity, the distance between target and background and atmospheric extinction coefficient, and the distribution of correction coefficient has a certain regularity. In order to meet the needs of visibility observation business, the results of visibility observation under non-ideal conditions need to be modified, especially when the correction coefficient is less than 0.8.