The invention provides a control method and a 
system of 
electricity theft detection used for a medium and 
low voltage mixed power distribution network. The method includes: in the selected medium and 
low voltage mixed power distribution network, a set M including n to-be-detected nodes is set, a to-be-detected node Mi is selected and measured, 
power index values of the medium and 
low voltage mixed power distribution network are obtained, the weight of a measuring value is set, state 
estimation calculation of the medium and low 
voltage mixed power distribution network is conducted, and an 
active load estimation value of Mi is obtained; the 
active load estimation value of Mi and the practical measuring value are compared, if an absolute value obtained by subtracting the practical measuring value from the 
active load estimation value is greater than a preset 
detection threshold, it is determined that Mi has 
electricity theft behaviors; and another Mi is selected from the set M, the above operation process is repeated, and the process is stopped until the determination of 
electricity theft behaviors of all Mis in the set M is accomplished. According to the method and the 
system, problems that the analysis results of electricity theft are inaccurate and users with electricity theft cannot be discovered via the prompts of a meter device or regular manual inspection at the spot are solved.