The invention relates to a preparation method of a composite modified Y molecular sieve. The preparation method includes: firstly subjecting an NaY molecular sieve slurry to exchange with a rare earth solution under a temperature of 5-90DEG C and pH of 3.0-7.0 for 0.1-1.0h, then performing mixing and blending with a silicon-containing solution for 0.1-1.0h, then adding a soluble phosphorus-containing compound, carrying out stirring, filtering, washing and drying, and performing roasting for 0.1-3.0h under 100-700DEG C and 100% water vapor conditions; and subjecting the roasted molecular sieve to exchange with an ammonium salt solution, filtering, washing and drying, thus obtaining the composite modified Y molecular sieve. The molecular sieve obtained by the method has good pore structure and pore size distribution. At the same time, when the silicon-aluminum ratio of the molecular sieve is improved, the crystal retention of the molecular sieve is improved, and then the activity and hydrothermal stability of the molecular sieve are improved.