A spatial decomposition model and solution of reliability index based on investment optimization are proposed. Associate the reliability indicators of the entire network with the four specific business indicators related to power supply reliability in each region. Realize the spatial decomposition of the reliability index, so as to construct the objective function and list the constraint equations. Calculate the cost of each regional business index for each corresponding reduction in the number of households, and determine the investment sequence in the order of low to high cost-efficiency ratio of each index in each region within the entire network to solve the model. The comparative analysis through calculation examples shows that the space decomposition model and its solution of reliability index based on investment optimization proposed in this paper is effective, it can reduce redundant investment, overcome the blindness when determining the reliability improvement target of each region, and is conducive to reliability refined management and precise investment.