The invention introduces a method for calculating floating-point operands, which contains at least the following steps: receiving an FP (floating-point) operand in a first format from a source register, wherein the first format is one of a group of first formats of different kinds; converting the FP operand in the first format into an FP operand in a second format; generating a calculation result in the second format by calculating the FP operand in the second format; converting the calculation result in the second format into a calculation result in the first format; and writing-back the calculation result of the first format.