The invention discloses a lanthanum-containing X type molecular sieve and a preparation method thereof, belonging to the field of molecular sieve materials. The preparation method comprises the following steps of: mixing the X type molecular sieve raw powder with an HNO3 aqueous solution and stirring; standing, filtering and drying; mixing the obtained powder with a La(NO3)3-containing HNO3 aqueous solution and stirring; transferring into a polytetrafluoroethylene reaction kettle and performing hydrothermal treatment; and after reaction, filtering the product, drying and roasting to obtain a lanthanum-modified X type molecular sieve. In the method, the exchange degree of La ions is high; after the ion exchange, the skeleton structure of the X molecular sieve is kept good; and moreover, repeated ion exchange is not needed, the technology is simple, and large-scale preparation is easy to implement.