The invention belongs to the technical field of fluorescent probes for 
metal ion detection and in particular relates to a fluorescent 
molecular probe for Cu2+ detection as well as a preparation method thereof and an application thereof. According to the invention, 
diaminomaleonitrile, which is modified at the fourth bit of N-n-butyl-1,8-naphthalimide in the form of 
Schiff base, is adopted as a probe molecule. The probe molecule has no 
fluorescence in an 
acetonitrile solution; Cu2+ 
aqueous solution of less than 1 equivalent is added to the probe solution; due to effects of amino groups and imino groups in the Cu2+ and the probe molecule, 
photoinduced electron transfer (PET) process is blocked while being excited by 360 nm lights; the 
absorbed energy is used for emitting lights so as to emit green 
fluorescence; when the concentration of the Cu2+ is more than 1 equivalent, the Cu2+ begins to act on together with two cyano groups so that the 
electron withdrawing capability is enhanced and the emission 
wavelength shifts to blue. With the increasing of the concentration, 
green light gradually disappears and the 
blue light is gradually enhanced. No corresponding response is generated by adding other 
metal ions. According to the invention, the 
copper ions are specially recognized by using the probe molecule; and with the increasing of the Cu2+ concentration, the probe is correspondingly changed from no 
fluorescence, to green fluorescence and to blue fluorescence. The special 
advantage of 
concentration response obtains broad application prospect in the particle detection field.