The invention relates to an experiment device used for monitoring formation of an artificial gas cap. The experiment device comprises a rotary device, a reaction kettle, an injection system, a sampling and analyzing system and a constant-temperature heating device. The rotary device and the reaction kettle are arranged in the constant-temperature heating device. The injection system and the sampling and analyzing system are connected with the reaction kettle. Long-strip-shaped visual windows are formed in the front side and the rear side of the upper half portion of the reaction kettle respectively. At least six sampling ports distributed at equal intervals are formed in the left side and the right side of the upper half portion of the reaction kettle respectively. An injection tubular column is arranged in the reaction kettle, the top end of the injection tubular column is connected with an injection port, and the bottom end of the injection tubular column is located in the center of the reaction kettle. Sampling tubular columns are inserted into the left side sampling ports and the right side sampling ports respectively, one end of each sampling tubular column is located in the reaction kettle, and the other end of each sampling tubular column is located outside the reaction kettle. By means of the experiment device, the formation process, the scale and the shape of the artificial gas cap under the high-temperature and high-pressure environment can be observed through the visual windows. Meanwhile, gas-oil ratios and gas compositions of fluid in the sampling tubular columns in different positions in the reaction kettle can be quantitatively analyzed.