The output shock wave pressure attenuated by different thicknesses of PMMA gaps of booster JO with small size charge diameter was measured by manganin piezoresistance method. The exponential regularity of the shock wave attenuation coefficient versus the charge diameter in PMMA was obtained by fitting experimental data. The load density of the booster was 90% of the theoretical density, and the charge diameter was 1.5 mm, 3 mm, 5 mm and 8 mm with confinement of 45^# steel and PMMA. Results show that the attenuation coefficient tends to decrease with the increasing of the charge diameter under the same confinement. The confinement of the charge is stronger,the change is more obvious.