The sudden change of the boundary of small signal stability region（SSSR） is shown through the studies of a 4-generator 11-bus power system models. It is found that this type of sudden change is due to jumpings of the critical oscillation mode between different modes on the corresponding boundary of SSSR. Meanwhile， it is illustrated that there is other mechanism that may result in this sudden change and it could be concluded that jumpings of critical oscillation mode is only one of the causes that lead to drastic changes of the boundary's shape of SSSR.