Dual-injection strategy combined with dual-fuel can provide flexible and instantaneous blend ratios according to engine demands under different operating conditions. Furthermore, ethanol direct injection has the potentials to increase the engine compression ratio and thermal efficiency by taking advantages of ethanol fuel such as the high octane number and latent heat. Therefore, in order to maximize the charge cooling effect, a dual-fuel (gasoline and ethanol) dual-direct injection system is proposed and investigated in this paper. In current work, different injection modes in terms of knock suppression and combustion performance are compared and studied on a single-cylinder SI engine with stoichiometric air/fuel ratios. The results show that the new injection strategy of dual-fuel dual-direct injection mode can significantly expand engine load compared with conventional modes of gasoline direct injection plus ethanol port injection and ethanol direct injection plus gasoline port injection. Direct injection ethanol mode can reduce knocking tendency. However, with higher brake mean effective pressure, dual-fuel dual-injection mode shows higher knock tendency. Dual-fuel dual-injection mode can increase brake mean effective pressure and thermal efficiency simultaneously. However, the brake thermal efficiency behavior is non-linear when increasing brake mean effective pressure and direct injection ethanol mass fraction. The equivalent heat value brake specific fuel consumption decreases with the increase of direct injection ethanol ratio, and dual-fuel dual-injection mode has superior fuel conversion efficiency. Relatively high maximum pressure can be found in dual-fuel dual-injection mode. With the increase of ethanol ratio, the combustion phasing is further advanced, resulting in better combustion, which contributes to higher thermal efficiency.