The invention discloses a preparation method and application of a medium/low-temperature environment-friendly crosslinking agent, belonging to the technical field of oilfield chemistry. The preparation method of the crosslinking agent comprises the following steps: adding an aldehyde solution into a reaction vessel, and regulating the pH value of the solution to 8-9; adding a stabilizer, and performing uniform stirring; and finally, adding melamine, performing heating to 70-75 DEG C, and performing a sufficient reaction while keeping at such temperature, thereby obtaining the medium/low-temperature environment-friendly crosslinking agent, wherein the mass percents of the aldehyde solution, stabilizer and melamine are respectively 20-25%, 20-25% and 50-60%. When being used for preparing a crosslinked polymer gel profile control agent, the medium/low-temperature environment-friendly crosslinking agent prepared by the method can form a gel at 40-60 DEG C, and the gel formation time is controllable within the range of 12 hours to 7 days; the formed gel system is stable, and the gel viscosity can reach 8000 mPa.s or above; and the crosslinking agent is safe and nontoxic, has small pollution to the oil reservoir, and satisfies the requirements for safety and environment friendliness in the production process.