Numerous studies have shown that oxygen vacancies play an important role on the formation of two-dimensional electron gas (2DEG) at SrTiO3-based heterointerfaces. Previously, it is widely believed that the main mechanism is that the oxygen vacancies in SrTiO3 directly contribute electrons to the 2DEG. Here, we performed transport measurements during the creation of 2DEG for depositing amorphous LaAlO3 on SrTiO3 substrates and related heterostructures. Our result suggests that, unlike the previous viewpoint, in this kind of 2DEG the determinant mechanism is the electron transfer from the oxygen vacancies in the film grown on SrTiO3, rather than the oxygen vacancies in SrTiO3 themselves. This effect is so striking that an amorphous film of less than 10% monolayer coverage on SrTiO3, or equivalently 0.04 nm, can already generate a highly conducting 2DEG. The present result may have a general implication and provide a possible way to understand the long-standing debate on the origin of 2DEG at SrTiO3-based heterointerfaces.