The variations of width and shift of Ar Ⅰ(2P2→1S5) spectral line with discharge parameters were studied in a slot microplasma. In order to measure the wavelength shift, the Ar Ⅰ(2P2→1S5) spectral line emitted from argon discharge at pressure of 10 Pa was used as a reference line. With the gas pressure increasing in the range of 1×104～6×104 Pa, the width and shift of Ar Ⅰ(2P2→1S5) spectral line were measured in argon (99.92%)/air discharge. It was found that both the width and the shift of Ar Ⅰ(2P2→1S5) spectral line increase linearly with the increase in gas pressure, indicating that the electron density increases with the increase in gas pressure. In addition, the width and the shift of Ar Ⅰ(2P2→1S5) spectral line at gas gap width of 100 and 300 μm were measured for comparison. It was found that both increase with the increase in gas gap width, which indicates that the electron density in the slot microplasma increases with the increase in gas gap width.