The present invention provides a Y molecular sieve and a preparation method thereof, wherein the unit cell parameter of the molecular sieve is 24.35-24.55 angstrom, the Al distribution parameter D ofthe molecular sieve is more than or equal to 0.3 and is less than or equal to 0.8, the mesoporous volume of the molecular sieve is 0.15-1.0 ml/g, the volume of the mesopores of the molecular sieve accounts for 30-70% of the total pore volume, the amount of the strong acid of the molecular sieve accounts for 30-60% of the total acid amount, and a ratio of the acid B to the acid L is 20-100. According to the present invention, with the application of the Y molecular sieve as the active component to prepare the catalyst, the excellent conversion ability, the high middle distillate yield and the low coke yield can be provided in the catalytic cracking and the hydrocracking of heavy oil.