Off-model calculation is a mathematical term, namely recursive calculation. Off-balance-sheet calculation is an operation that completely writes out the calculation process, that is, the calculation that deviates from the vertical type. It is a basic concept in mathematics, which is mainly used for four operations and simple operations. Off-type calculation is widely used, which can be used for integer, decimal and fractional operations, as well as algebraic operations. Without brackets, multiply and divide first, then add and subtract. In the case of brackets, the expressions in brackets are calculated first, and the calculation process is simplified by using the laws of additive commutative law, associative law, multiplicative commutative law, associative law and distributive law. In the process of writing off-line calculation, we should pay attention to the alignment of equal signs and gradually show the calculation process of each step.