A mining system of a marine non-diagenetic natural gas hydrate reservoir and a mining process are disclosed. The system comprises a drilling platform (1), a shaft access channel (2), and a main production channel (3) all sequentially connected. Two sides of the main production channel (3) are provided with mine accessways (4) communicating with the main production channel (3). In the main production channel (3), mining is performed by using a mining device (5). Mining is performed in the mine accessways (4) by using a mining device, and the mine accessways (4) are backfilled by using a backfilling device (6). According to the mining process, the shaft access channel (2) is built, the main production channel (3) is built, and the mining device (5) and the backfilling device (6) are installed. A hydrate ore body is mined in branch accessways. Secondary crushing and decomposition are performed. Separated natural gas is delivered to the drilling platform (1). Remaining slurry is mixed with carbon dioxide to make solid carbon dioxide hydrate with which the excavated accessways (401) are backfilled. This cycle is repeated until all production seams are completely mined. The mining system and the mining process of the invention maintain the stability of the submarine geological structure and avoid risk of seabed subsidence, collapse, and landslide likely caused by conventional mining methods.