With the development of energy technology, the extended planning of integrated urban electricity and natural gas systems has become a hot issue. In recent years, the frequent occurrence of extreme natural disasters threaten the operation of the energy system. Under the background, the extended planning problem of integrated urban electricity and natural gas systems with consideration of resilience upgrading is addressed in this paper. Firstly, the basic components in the system are divided into disaster resistance levels to describe the damage of components after disaster in the extreme natural disaster scenario. Then, a robust optimization model of urban power gas interconnected energy system expansion planning considering elastic lifting is proposed to to minimize the sum of investment cost and load shedding cost by using energy storage batteries, distributed gas generators, gas storage tanks, power to gas devices and other equipment. Finally, the column and constraint generation algorithm are utilized to solve the proposed model. In addition, the improved IEEE 33 bus power system and Belgium 20 bus power system are chosen for simulation test. The planning results verifies that the proposed extended planning model can improve the system flexibility.