The invention relates to a preparation method of a soil heavy metal passivant, and a soil Pb passivation method. The preparation method comprises the following steps: 1, adding an alkaline solution and sieved coal ash to a reaction vessel in proportion, carrying out ultrasonic dispersion stirring to obtain a slurry, and carrying out a hydrothermal crystallization reaction through putting the reaction vessel in an oven; and 2, cooling and centrifuging products obtained after the reaction, washing the products with deionized water, drying, grinding to obtain synthetic zeolite powder, burning rice husk at normal pressure to prepare rice husk ash, and uniformly mixing straw powder, the coal ash, the synthetic zeolite powder and the rice husk ash according to weight percentages to obtain the heavy metal passivant. The loess farmland soil Pb passivation effect can be improved through adjusting conditions comprising the soil pH value, the soil water content, the passivant quality, the passivation time, the passivation temperature and the like. The passivation method has the advantages of low cost, simple operation, strong practicality, good safety, and easy popularization, can passivate soil heavy metals and also can improve the soil properties, so the passivation method has potential application values in soil heavy metal pollution restoration.