The particle-number conserving (PNC) method in the framework of cranked shell model (CSM) is developed to deal with the reflection-asymmetric nuclear system by applying the $S_x$ symmetry. Based on an octupole-deformed Nilsson potential, the alternating-parity bands in element{236,238}{U} and element{238,240}{Pu} are investigated. The experimental kinematic moments of inertia (MoI) and the angular momentum alignments of all studied bands are reproduced well in the PNC-CSM calculations. The striking difference of rotational behaviors between U and Pu isotopes can be linked to the strength of octupole correlations. The upbendings of the alternating-parity bands inelement{236,238}{U} are due to the alignments of pairs of nucleons occupying $ u g_{9/2}$, $pi f_{7/2}$ orbitals and $ u j_{15/2}$, $pi i_{13/2}$ high-$j$ intruder orbitals. Particularly, the interference terms of nucleon occupying the octupole-correlation pairs of $ u^2 j_{15/2} g_{9/2}$ and of $pi^2 i_{13/2} f_{7/2}$ give a very important contribution to the suddenly gained alignments.
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