Spectral hole depth dependence on optical length in Tm³⁺:YAG was theoretically and experimentally investigated. A novel model was proposed to analyze the spectral hole depth dependence on optical length. With the proposed model, the spectral hole depth dependence on optical length had been derived theoretically. According to the theoretic result, when temperature was higher than 4 K, the maximum spectral hole depth could be found by choosing the optimum optical length. The theoretical analysis were experimentally tested using spectral hole burning in Tm³⁺:YAG for a couple of configurations and sizes of the beams.