In this study, we systematically investigated the removal of low-concentration formaldehyde by the fiber optic illuminated honeycomb monolith photocatalytic reactor and the influential factors including formaldehyde concentration (0.8–2.0 ppm), relative humidity (RH30–70%), and air flow rate (800–1600 mL/min). The experimental results of various formaldehyde concentrations indicate the kinetic fits the Langmuir–Hinshelwood model, and the rate constants (k) under RH30%, 50% and 70% are 1.09, 1.37, and 1.68 μ-mole/m2/s, respectively. The Langmuir adsorption constants of formaldehyde under RH30%, 50% and 70% are 4.10 × 10–3, 2.98 × 10–3 and 2.14 × 10–3 ppm–1, respectively. Increasing relative humidity has a positive effect on the rate constant of photocatalytic oxidation k, which is relevant to the enhancement effect of relative humidity on the formation of hydroxyl radicals. On the other hand, the formaldehyde conversion and Langmuir adsorption constant of formaldehyde decrease with the increase of relative humidity, which may be associated with the competition between formaldehyde and water molecules for the adsorption sites on the surface of TiO2 photocatalyst. Because the air flow rate was low (≤ 1600 mL/min), the gas retention time (≥ 7.7 sec) was long enough for the reactor to achieve a high formaldehyde conversion (≥ 92%), but the breakthrough of formaldehyde might occur when air flow rate > 4200 mL/min.