The site preference of Si in TiAl3 is calculated by using a first principles method based on density functional theory. The analyses of the c/a value， the formation energy and density of states of systems with different substitution behaviours show that Si prefers to occupy the Al site. The site preference is primarily determined by the electronic structure， and the calculated result is in good agreement with the experimental result. In order to further study the site preference， the occupied sites are divided into two different distributions， the loose distribution and the compact distribution. The calculations of total energy， the formation energy and density of states of systems indicate that Si has a preference for Al（2） sites with a loose distribution. When the systems with Al（2） sites are substituted， the value of c/a increases with doped Si concentration increasing， which is opposite to the case where Al（1） sites are substituted. The study also indicates that the limited solubility of Si in TiAl3 lies between 12.5at% and 18.75at%.