The invention relates to synthesis and application methods of a halogen-free and formaldehyde-emission-free cotton permanent flame retardant of a nitrogen-phosphorus collaboration system. The synthesis method is characterized by comprising the following steps of (1), putting N,N-methylene bisacrylamide into a four-mouth flask, heating, melting, then putting dimethyl methylphosphate, meanwhile, adding a catalyst dibutyltin dilaurate or sodium methoxide, raising a temperature to 75 to 80 DEG C while agitating, carrying out heat preservation, reacting for approximately 2 to 3 hours, and cooling to a room temperature to obtain an intermediate A; (2), putting the obtained intermediate A into the four-mouth flask, adding an aqueous solution, firstly regulating pH (potential of Hydrogen) to be between 8 and 9 by using triethanolamine, raising the temperature of a reaction solution to 85 to 90 DEG C, slowly dropwise adding 1,4-dichlorobutane while agitating, carrying out the heat preservation,and reacting for 2 to 3 hours, so that a final product B can be obtained. The halogen-free and formaldehyde-emission-free cotton permanent flame retardant has the characteristics of being simple in process, high in flame retarding efficiency and a few in the influence on the strength and the hand feeling of a textile.