Since the distributed solid electric thermal storage (DSETS) is affected by various environmental temperature factors, the schedule of energy transfer may be uncontrolled, which causes the security risks in the operation of multi-energy systems with DSETS. Thus, to tackle this problem, a security assessment and preventive control method of combined electricity and heat systems with DSETS is proposed. Firstly, a two-stage optimization model is established. The objective of the first stage is to minimize the system operation cost. A contingency set is established for the combined electricity and heat systems with DSETS. The contingency set is used to simulate the disturbance of the DSETS’ load increase and the branch N-1, and the proposed evaluation index of branch overload is applied to evaluate the security of the system after a contingency. The preventive control is adopted in the second stage when the branch is overloaded in the first stage of optimization. The objective of the second stage is to minimize the active power adjustment of each unit of the system, and the overload branch is corrected. The active power flow of the branch is reduced below the safety margin by regulating the output of each unit of the system. Finally, by comparing the safety of systems with or without the preventive control, the validity of the preventive control model and evaluation index is verified. The overload indices of the above two contingencies are 2.82% and 6.96%, respectively, and the overload of a branch is corrected by adopting the preventive control.