The S-0, T-1, and S-1 potential energy surfaces for the HCOOH dissociation and isomerization processes have been mapped with different ab initio methods. The wavelength-dependent mechanism for the HCOOH dissociation was elucidated through the computed potential energy surfaces and the surface crossing points. The HCOOH molecules in S-1 by excitation at 248 nm mainly decay to the ground state via the S-0 and S-1 vibronic interaction, followed by molecular eliminations in the ground state. The S-1 direct dissociation to HCO((2)A') + OH ((2)Pi) is the dominant pathway upon photoexcitation at 240-210 nm. Meanwhile, there is a slight probability that the system relaxes to the ground state via the S-0 and S-1 vibronic interaction at these wavelengths. After irradiation of HCOOH at 193 nm, the S-1 direct dissociation into HCO((2)A') + OH((2)Pi) is energetically the most favorable pathway. In view of high 1C efficiency at the S-0/S-1 conical crossing, the S-1 --> S-0 internal conversion via the S-0/S-1 point can occur with considerable efficiency. In addition, the S-1 isomerization probably plays a dominant role in the partially conformational memory of the HCOOH photodissociation, which has been discussed in detail.