There are three models to define investment in the investor-state arbitration mechanism, which include asset-based model, enterprise-based model and hybrid-based model. Based on applying these models, the arbitral awards explain technical ways used by contracting parties to limit the meaning of investment, such as, the expression of assets in accordance with the domestic law refers only to the validity of the investment; investments in the territory of the contracting parties do not require all transactions to be completed in the territory of the contracting parties. The expression of assets directly or indirectly, owned or controlled would protect benefit of minor shareholders and indirect shareholders. China is the top destination of foreign investors and one of the world largest investors. It is necessary for China to upgrade the existing asset-based model to hybrid-based model of defining investment and adjust the restrictive interpretations of investment in international investment agreements accordingly.