In this study, the aim is to investigate the internal flow characteristics related to the oxidizer fuel (O/F) ratio of a recessed swirl coaxial injector for gas generators and main combustors in liquid rocket engines. The mixing process of internal flow is very important in a recessed swirl coaxial injector; however, measuring internal flow is almost impossible except if using numerical analysis. This paper describes the design of a swirl coaxial injector that enables flow visualization and film thickness measurement and gives the results of cold flow tests using water as a simulant fluid. The injection conditions in the injector for the gas generator and main combustor were set to O/F ratios of 0.32 and 2.44 and the off-design points were set to 10% and 15%, respectively. Recess length varied by 1 mm for each case. The liquid film thickness variations after collision between two liquid flows were measured using the electrical conductance method. In addition, the characteristics of liquid film wave frequency and amplitude were analyzed based on liquid film thickness measurement and mixing time. This study confirms that liquid film becomes thinner after the mixing process. This is based on visualizing the internal flow and measuring liquid film thickness. The degree of the thinning of the liquid film varies with the momentum flux ratio according to the O/F ratio. An unstable internal flow was detected in the mixing process near the injector tip position.