Disclosed are a sulfide-based solid electrolyte imparted with improved lithium ion conductivity and a method of preparing the same. More particularly, disclosed is a sulfide-based solid electrolyte containing a lithium element (Li), a phosphorus element (P), a sulfur element (S) and a halogen element (X), and including a crystal phase of an argyrodite crystal structure, wherein a molar ratio (X/P) of the halogen element (X) to the phosphorus element (P) is higher than 1.