This paper derives the mathematical adjoint equation of neutron transport equation, which is solved in hexagonal geometry by interface current method in the condition that the neutron source in cell adopts constant assumption. The calculation method of mathematical equation and the progress of inner and outer iteration are introduced. Based on the TPHEX code for forward equation calculation, the TPHEX_J0 code to calculate the mathematical equation is encoded. The calculation checkout proves that the keff of system calculated by TPHEX_J0 and TPHEX code are all in good agreement, and the calculation results of TPHEX_J0 code are reliable.