Based on the 1 550 nm single-frequency Erbium-Ytterbium co-doped fiber amplifier (EYDFA), the linewidth broadening with different seed powers and gain fiber temperatures was experimentally investigated. The results demonstrate that, to obtain the same output power, the increase of the seed power was beneficial to improve the output signal to noise ratio(SNR), and to reduce the linewidth broadening. The increase of gain fiber temperature can improve slope efficiency and enhance the amplified spontaneous emission (ASE) intensity but broaden linewidth. Meanwhile, the reason of linewidth broadening was discussed and the ASE was considered as one of the reason of linewidth variation.