This kind of arbitrage in the problem belongs to a vertical arbitrage of options. Vertical arbitrage of put options refers to selling put options at a lower strike price and buying put options with the same term at a higher strike price. It can be analyzed in this way, regardless of other expenses, assuming that the price of 1 and the Hang Seng Index X fall below X≤ 10000 when the contract expires, then both contracts will be executed. Total income = (10200-x)+(x-10000)+(120-100) = 220.2. Hang seng index price 10000 < x ≤ 1000. Total income = (10200-x)+(120-100) =10220-x, because10000 < x ≤10220. Total income = 120- 100=20. It can be seen that the purpose of vertical arbitrage of put options is to make profits in a bear market, and its maximum possible profit is (higher strike price-lower strike price+net commission). In addition, it is worth noting that the problem itself should be problematic. As the analysis shows, no matter how the price changes, investors can have stable positive returns, which is impossible under the condition of complete market, because once this completely risk-free arbitrage exists, it will soon be discovered by investors. The problem is that under the same term, the premium of put option with higher exercise price should be higher than that of put option with lower exercise price, which is also in line with the principle of reciprocity of risk and return.