The invention discloses a calculating method for a cross-quadrant attitude angle based on quaternion. The calculating method comprises the following steps: respectively calculating and acquiring a pitching angle initial value, a yaw angle initial value and a roll angle initial value; respectively calculating and acquiring a yaw angle median value in the current period and a roll angle median value in the current period; determining the counting of the yaw angle; determining the counting of the roll angle; calculating and acquiring a yaw angle final value in the current period; calculating and acquiring a roll angle final value in the current period; calculating and acquiring a decoupling matrix of the yaw angle in the current period; calculating and acquiring the decoupling matrix of the roll angle in the current period; determining a quaternion transition matrix in the current period; calculating and acquiring a pitching angle transition matrix in the current period; calculating and acquiring a quaternion corresponding to the pitching angle in the current period; calculating and acquiring a pitching angle median value in the current period; determining the counting of the pitching angle; and calculating and acquiring a pitching angle final value in the current period. According to the calculating method, decrease in singularity and precision caused by the traditional resolving method in the prior art can be avoided, so that the resolving precision can be ensured.