Within the framework of the quantum molecular dynamics transport model, the pion production and isospin effect in heavy-ion collisions have been thoroughly investigated. The energy conservation in the decay of ∆ and reabsorption of pion in nuclear medium are taken into account. The influence of the stiffness of symmetry energy and the pion potential on the pion production are systematically investigated and compared with the FOPI data via the transverse momentum, longitudinal rapidity and collective flow in collisions of 197 Au + 197 Au. It is manifested that the pion yields are slightly suppressed in the domain of mid-rapidity and high momentum. The antiflow phenomena is reduced by implementing the pion potential and more consistent with the experimental data. The isospin diffusion in the high-density region (1.2ρ0 -1.8ρ0) is investigated by analyzing the neutron/proton and π − /π + ratios in the reaction of 132 Sn + 124 Sn at the incident energy of 270 MeV/nucleon. A soft symmetry energy with the slope parameter of L(ρ0) = 42 ± 25 MeV is obtained by analyzing the experimental data from the SπRIT collaboration.