The invention discloses an Alpha phase polymorphism of an N6 type mononuclear Fe(II) spin crossover complex. The crystal parameters are as followings: the chemical formula is C18H16Br2FeN10; the molecular weight is 588.08; the crystal system is an orthorhombic system; the space group is Pcan; a is 8.707(3) angstrom; b is 14.128(4) angstrom; c is 16.875(5) angstrom; the number of alpha, beta and gamma is equal to 90 degrees; the volume is 2075.8(11) angstrom; Z is equal to 4. The preparation process is relatively simple and convenient; a monocrystal capable of being used as a core sensor element in a temperature sensor can be generated; variable temperature magnetic susceptibility tests show that the magnetic susceptibility of the Alpha phase crystal has an obvious mutation in a position mear 150K along with the temperature reduction; in the temperature rise and temperature reduction mode, curves are not completely coincident; two different stable states exist at the same temperature; acertain memory effect is achieved in the previous experienced process of the system; the memory characteristics provide possibility for the application of the Alpha phase polymorphism in the aspectsof molecular switches, molecular storage and the like.