1-Butene polymerizations were carried out by using a $MgCl_2/TiCl_4/ID-AlR_3$/ED catalyst system in n-hexane, therein the internal donor (ID) is diisobutyl phthalate (DIBP), the external donor (ED) is diphenyl dimethoxysilane (DDS) or cyclohexyl methyl dimethoxysilane (CHMMS), and the $AlR_3$ activator is $AlEt_3$ or $Al(i-Bu)_3$. The effect of the molar ratio of DIBP/Mg and polymerization conditions on the catalytic activity and the isotactic index (II) of poly(1-butene) (PB) was investigated. The dosage of DIBP remarkably improved the catalytic activity and the II of PB. CHMMS was more effective than DDS for increasing the II of PB. The addition of a small amount of hydrogen enhanced the catalytic activity from 15.1 to 17.5 kg-$PB{cdot}gTi^{-1}$ and the II value of PB from 89.3% to 95.1%. The mixture of $AlR_3$ was beneficial to increase the catalytic activity with negligible effect on the isospecificity.