A method of measuring the exchange constant J of two-qubit system is proposed. The energy levels and eigenvectors are calculated by adiabatic approximation. The length of polarization of this system is obtained,and the value of exchange constant of symmetric system is found to equal to the value of z component of external magnetic field at the level crossing. In addition,the concurrence of each eigenvector accords with the measurement of Von Neumann entropy for symmetry system.