1. A method adapted to a particle analyzer for discriminating particle groups, the method comprising: generating, based on characteristics of particles and particle count within a sample to be measured, a particle characteristic distribution histogram, wherein a first direction coordinate of the particle characteristic distribution histogram indicates channels for representing the characteristics of the particles, and a second direction coordinate indicates the particle count; setting a valid area selection height in the particle characteristics distribution histogram, and generating an equivalent negative histogram based on the set height and the particle characteristic distribution histogram, wherein a height value of a bar in the equivalent negative histogram for a certain channel equals the valid area selection height minus a height value of a bar in the particle characteristic histogram for said channel; processing the equivalent negative histogram by using an equation M k = ∫ x · g ⁡ ( f ⁡ ( x ) ) ⁢ ⅆ x ∫ g ⁡ ( f ⁡ ( x ) ) ⁢ ⅆ x to obtain a dividing line between the particle groups, where M k is the dividing line between the particle groups; x is a channel value in the equivalent negative histogram; f(x) is a height value of a bar in the equivalent negative histogram for channel x; and g(•) is a processing function with the following features: 
 ∀x≧y≧0, g ( x )≧ g ( y )≧0 
 ∀x<0, g ( x )=0.