Following a discussion of the affiliation of stability and security， it is pointed out that the static security constraints， transient stability constraints and whole process security assessment should be dealt with under different procedures in power grid planning. The procedure of security assessment for power grid planning under high penetration of variable generation is presented. It is pointed out that part of the courses of the cascading events is typical of the open complex giant system and should be simplified since it cannot be exactly simulated. The other part of the courses is with strong cause and effect relationship between the preceding and succeeding events and can be simulated by improved simulation software. By taking into account the pre-fault states of the generation system， the whole process risk assessing method is presented. With the method， the costs of stabilizing facilities and the power cut losses will be included in the composite annual costs for comparing and choosing the planning schemes.