The preparation of $YBa_2Cu_4O_8$ thick film on Ag substrate by electrophoretic deposition was directly studied. Electrophoretic deposition was carried out in solution, which was composed of presintered $YBa_2Cu_4O_8$ powder, sodium, and 2-propanol as a solvent. The deposited thick films were heat-treated in $O_2$ at 815$^{circ}$C for 12 hours and at 450$^{circ}$C for 12 hours. We succeeded in obtaining superconducting $YBa_2Cu_4O_8$ films on Ag substrate. Even though the chemical reactions at the $YBa_2Cu_4O_8$/Ag interface occurred in the range of 10 ${mu}$m, superconductivity was not depressed. The thickness of the films was in the range of 60-80 ${mu}$ m. The characteristics of the films were examined by electric resistance measurements, X-ray diffraction, and SEM observations.