The invention discloses a variable-field scanning 
microscopic method based on a fixed 
optical path system, and a device thereof. At first, a beam expanding 
system is used for uniformly expanding 
laser beams which are transmitted from a 
laser so as to form uniform 
laser beams; then a two-dimensional space 
discrete system is used for the two-dimensional space 
discretization of the uniform 
laser beams so as to form a sub-beam of M multiplied by N; the sub-beam of M multiplied by N orderly passes through a collimating lens, a scanning 
galvanometer, a reflector, a dichroic mirror and a 
fluorescence microscope, and then focuses on the sample on a sample platform; thus the excited 
fluorescence can be formed on the sample; the 
fluorescence microscope is used for collecting the excited fluorescence, which is transmitted into an imaging 
system after being reflected by the dichroic mirror and focused by a focusing lens. The scanning of the 
galvanometer and the crossing movement of the sample platform are combined: the scanning step length and the scanning 
step number of the 
galvanometer are changed; the area of the unit viewing field is used as the step length to move the sample platform; thus the scanned viewing field can be changed. The scanning 
microscopic method realizes the flexible switching between the low-resolution large viewing field and the high-resolution small viewing field, and satisfies the requirements in different biomedical applications.