The emergency support systems are popularly considered and developed at home and abroad recently in order to deal with the nuclear emergency, such as SESAME France, SEMAX from Korea. An emergency support system usually includes these functional units: nuclear condition monitoring, initiating event diagnosis, break size calculation, instrument availability judgment, accident simulation, accident prognosis and response support. Here, the diagnosis part is mainly concerned, including a nuclear condition monitoring unit, initiating event diagnosis unit, and break size calculation unit. In this paper, firstly, the emergency support system developed by us will be introduced briefly. Secondly, a nuclear condition monitoring method will be introduced. Then, an integrated initiating diagnosis method (including some different diagnosis methods and results fusion method) and two break size diagnosis methods for nuclear emergency are showed and described in detail. At last, a case of LOCA accident is used for the verification of these diagnosis methods and some analyzing results and conclusions are given. The results show that these methods mentioned above are very useful for dealing with the nuclear emergency of nuclear power plant (NPP), which can offer much information (such as the initial event type, the break size and instrument availability etc.) for operators.