Based on the ring-shaped laser-diode array side-pumped Nd:YLF high gain amplifier and the conventional plan-concave resonator, a simple structure single-longitudinal-mode single-pulse Q-switched oscillator is demonstrated by taking advantages of mode-selecting property of passive Q-switches (Cr4+:YAG crystal) and etalons (formed by two end of uncoated Nd:YLF rod). The average pulse width is 9.3 ns, the average energy is 1.218 mJ, the pulse width stability is 8% by RMS and the pulse energy stability is 3% by RMS and the output pulse reaches the peak-power of 0.13 MW. This kind of oscillator holds the excellent single-longitudinal mode probability great than 99% in long running time. Keep in mind of its simplicity and unsensible to the change of working condition, it is preferred to put this kind of oscillator into practical usage in these situations where accurate time sequence is not required.