In order to study the enhancement effect of hemispherical cavity on plasma emission, we use hemispherical cavity with different diameters to confine the plasma generated by laser ablation alloy steel. It is found that the optimum enhancement factor of the hemispherical cavity on the plasma emission increases first, then decreases, then increases and finally decreases with the increase of the hemispherical cavity diameter. There is a linear relationship between the delay time corresponding to the best enhancement factor and the hemispherical cavity diameter. The results have shown that the hemispherical cavity has the best enhancement effect on the plasma emission when the diameter is 10 mm and the corresponding delay time is 10 μs. Spatial resolution measurement of the plasma emission spectra with and without hemispherical cavity are measured. It is found that the plasma without confinement evolved into an emission source with larger size and less intensity as a result of expansion. Whereas the plasma has a smaller size but higher intensity with hemispherical cavity confinement. It is demonstrated that the enhancement of the plasma emission is caused by the three-dimensional compression effect of the hemispherical cavity on the plasma, and the enhancement effect is affected by the diameter of the hemispherical cavity.