Modified molecular sieve characterized by improved sodium-resisting contamination activity and preparation method thereof are provided. The method comprises: adding molecular sieve in phosphorus-containing organic solution, and reacting for 10-200 minutes at temperature of 70-200° C. and pressure of 0.2-1.2 MPa, and then filtering, drying and calcining. The said modified molecular sieve contains 90-99 wt. % molecular sieve as dry basis and 1-10 wt. % phosphorus as oxide. The said method can improve the capability of sodium-resisting contamination effectively, and its technology is simple and fits the existing catalyst production apparatus and process. The said modified molecular sieve has high sodium-resisting contamination activity, and the model catalyst by sodium contamination has high activity retention.