The measurement of CO2 at air-sea interface is significant to marine scientific research. There are many commercial instruments can be used for CO2 measurements in marine environment, among which the instruments based on tunable diode laser absorption spectroscopy (TDLAS) play an important role due to the advantages of high sensitivity, good environmental adaptability, etc. As a core component of a TDLAS system, the volume and optical path of a multi-pass cavity limit the size and sensitivity of the system. For the application of underwater dissolved CO2 measurements, due to the samll quantity of degassing gas and high sensitivity, both the small size and long optical path are required for a multi-pass cavity. In this work, a miniature multi-pass cavity was designed for the potential underwater dissolved CO2 measurements. The miniature multi-pass cavity composed of two identical spherical mirrors (D=25.4 mm, f=50 mm)separated at a distance of 38 mm, realized 253 reflection times, providing an effective optical path length of 10 m and an inner volume of 90 mL. Based on the miniature multi-pass cavity, a direct absorption TDLAS system was developed for CO2 measurement. The system was evaluated with a series of different concentrations of CO2 standard gases. The obtained limit of detection (LOD) was about 26×10-6 (volume ratio), and the response was good linear with R2=99.986% over the whole range. A commercial ultraportable greenhouse gas analyzer (UGGA) from Los Gatos Research (LGR) was also used for comparison measurements, and the results showed a consistent trend with R2 of more than 97% under conditions of both high fluctuation at daytime and low changes at nighttime in laboratory. The experiments testified the performance of the developed TDLAS system for CO2 measurements, which means that the improved system will be used in field experiments in the near future.