SDS2 is a system to discover and identify the quantitative model consisting of simultaneous equations re
ecting the rst principles underlying the objective process through experiments. It consists of SSF and SDS, where the former is to discover the structure of the simultaneous equations and the latter to discover a quantitative formula of each complete equation. The power of SDS2 comes from the use of the complete subset structure in a set of simultaneous equations, the scale-types of the measurement data and the mathematical property of identity by which to constrain the admissible solutions. The basic principles, algorithms and implementation of SDS2 are described, and its eciency and practicality are demonstrated and discussed with large scale working examples. This work is to promote the research of scientic discovery to a novel and promising direction, since the conventional equation discovery systems could not handle such a simultaneous equation process in a highly ecient man...