A double-clad single-mode tellurite optical fiber (DC-SMTOF) with a loss of 0.5 dB/m @1545.1 nm was fabricated based on the TeO2–ZnO–Li2O–Bi2O3 (TZLB) material. During the fiber drawing process, a low level of residual birefringence was produced, which gave a chance for the investigation of the polarization modulation instability (PMI). With a nanosecond laser operated at 1545.1 nm as the pump source, the PMI effect in the DC-SMTOF was investigated using different fiber length and different average pump power. This investigation of the PMI generation in non-silica optical fibers may provide guidance for the generation of the highly repetitive optical pulse train.