The invention provides a synthesis method of p-hydroxybenzaldehyde. Phenol used as a raw material reacts with a Vilsmeier reagent to synthesize p-hydroxybenzaldehyde. The invention has advantages of easily available raw materials, short reaction period, simple operation, mild reaction conditions, safe reaction process and low requirement on equipment, and is more suitable for industrial production of p-hydroxybenzaldehyde compared to other synthesis method of p-hydroxybenzaldehyde; no by-product is generated in the synthesis process, and separation and purification in a post treatment process is simple, so as to lower the cost of post treatment, and enhance yield and purity of the target product p-hydroxybenzaldehyde, wherein the yield is higher than 90%, and the purity is above 98%; and the synthesis method resolves problems in existing phenol method for p-hydroxybenzaldehyde synthesis, such as difficulties in separation and purification in post treatment, low yield of p-hydroxybenzaldehyde and high cost.