It is vital to evaluate construction size of district power grid under the environment of electricity market. There are no comprehensive evaluation criteria or modeling methods that can represent the construction scale of distribution networks at present. On the basis of the actual conditions of the regional power grid， flow chart of assessing model is given. The macro-control index system reflecting the construction scale of distribution grid is then founded.When the maximum load and power supplies in the region area is given， the index model of transformer substation scale optimized is put forward based upon the principle of minimum overall expenditure in the supply district by mathematical programming， and each major indicator model is provided; the method of weighted average judging synthetically in terms of the experts’ appraisal is also provided. Verified via an actual distribution networks， the method proposed can be effective when used in assessing the construction scale of the regional distribution networks and guiding the scale planning of distribution networks.