Aiming at the defects of inconvenience and high cost of cold on-column sample injection operation in SH/T 0653-1998, the invention discloses a method of using a two-way valve to replace cold on-column sample injection, wherein the interior of the two-way valve is coated with inert substance. The method comprises the concrete operation modes: respectively preparing a mixed standard substance of n-alkanes with the carbon chain length ranging from n-C16 to n-C44, an n-C16 n-alkanes internal standard substance and cyclohexane solution of a petroleum wax sample; respectively putting in a gas chromatography for testing; calculating to obtain the mass percentage of each carbon number in the petroleum wax, the content of n-alkanes, the content of non-n-alkanes and the mass percentage of hydrocarbons containing 44 C or above. A gas chromatography sample injection mode is as follows: injecting a sample into an intermediate part of the two-way valve with a sample injection needle under the condition of closing make-up gas; connecting a chromatographic column from two ends of the the two-way valve; then opening the make-up gas; performing temperature programming for sample testing. The method is simple and easy to operate, the equipment cost is reduced, and the accuracy is equivalent to that of SH/T 0653-1998.