The invention discloses an indicator function-based fast 
decomposition method for a cross 
transfer function. The method comprises the steps of (1) obtaining a 
light source function J(f, g) of an imaging 
system; (2) dividing a region 
omega of describing a 
light source to obtain a group of orthogonal sub-regions omegaij and describing the 
light source function J(f, g) by using an indictor function Iij(f, g) on the orthogonal sub-regions; (3) calculating inverse 
Fourier transform of the indictor function Iij(f, g), obtaining a primary function Lij(x, y) in a 
spatial domain, projecting a light source mutual 
intensity function as shown in the specification in the 
spatial domain to a group of primary functions as shown in the specification and obtaining 
decomposition of the light source mutual 
intensity function as shown in the specification; (4) building a 
core function as shown in the specification of the cross 
transfer function as shown in the specification in the 
spatial domain; and (5)calculating a 
convolution of the 
core function as shown in the specification and a 
mask pattern as shown in the specification and obtaining an 
exposure pattern as shown in the specification on an 
image plane. The light 
source function is expressed by using the indicator function Iij(f, g) on the orthogonal sub-regions, the 
decomposition of the light source mutual 
intensity function as shown in thespecification is directly obtained by using an orthogonality relationship of the indicator function and a projection coefficient of the light source, the 
core function as shown in the specification of the cross 
transfer function as shown in the specification is easily obtained, so that 
light intensity distribution calculation is fast and efficient.