TiO2 is one of the most promising photoanodes for solar-hydrogen conversion by water splitting. Recently, hydrothermally synthetic rutile TiO2 nanorods (NRs) show outstanding photoelectrochemical (PEC) performance in water splitting because of its large surface area, fast carrier transport, and short diffusion length. However, light absorption and carrier transport conflict. Few have investigated the dependence of PEC performance on NR length. This study examines how different TiO2 NR lengths grown on an FTO substrate affects their PEC performance when splitting water. The results show that the optimal absorption length of rutile TiO2 NRs is 3.75 μm. However, under simulated solar illumination (AM1.5 G), the maximum PEC efficiency of these TiO2 NRs is 0.33% at a length of 500 nm. This suggests that carrier transport is the most important variable for improving PEC efficiency.