In this research, detailed characteristics of diesel spray and atomization based on marine engine scale have been studied under subcritical, transcritical and supercritical conditions. A large constant volume chamber with an inner diameter of 300 mm and a 300 pm single-nozzle injector are used to mimic the in-cylinder thermodynamic condition of marine diesel engine. Sub/trans/supercritical conditions are composed of nine cases by means of different diesel phase transition regions. Optical diagnostic measurements are conducted through backlight illumination technology and schlieren imaging technology in an inert atmosphere. The images are processed with multi-threshold technique to obtain quantitative spray parameters, such as jet and liquid penetrations, averaged lengths, liquid steady time, spray cone angles and R-parameter. Moreover, some new finding about the structure of spray under supercritical condition is also discussed. The results show that the spray liquid periphery is quite smooth and stable with shorter penetration under supercritical condition, meanwhile, the jet of spray behaves as thinner periphery near the injector nozzle region and shows lower density at downstream region. Higher ambient pressure produces a negative effect on the increase of the tip velocity and development of spray. With the transition from transcritical condition to supercritical condition, the liquid length decreases by 32 mm and there exists an obvious decrease of 35 mm from jet length to liquid length. A trade-off relationship between the jet cone angle and jet penetration is also found. Generally, this work is helpful to understand the detailed process of diesel atomization under transcritical and supercritical conditions.