and gravitational fields. These existing researches divide the electromagnetic and gravitational fields into two isolated parts, and then describe respectively the two fields. In the paper, the electromagnetic field and gravitational field can combine together to become one unitary field in the theoretical description, depicting the physics features of two fields simultaneously. (2) The preceding studies were difficult to unify similar physics quantities of two fields into the single definition. In the existing researches, the field potential (or field strength, field source) of gravitational field is different to that of electromagnetic field, and it is not able to unify them into the single definition. But the paper is able to unify the field potential (or field strength, field source) of electromagnetic and gravitational fields into the single definition.

In the paper, the author explores one new description method, introducing the complex quaternion space into the field theory, to describe the physical feature of electromagnetic and gravitational fields. This method’s inferences cover the most of conclusions of electromagnetic and gravitational fields described with the vector.

\[sec:level1\]Field Equations
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The octonion space $\mathbb{O}$ can be separated into two orthogonal subspaces, the quaternion space $\mathbb{H}_g$ and the $S$-quaternion space $\mathbb{H}_e$ . And the 