Based on the concept of an energy pump, water transportation in a carbon nanotube (CNT) is studied by molecular dynamics simulations. The influences of CNT pretwist angle, water mass, environmental temperature, CNT diameter, CNT channel length, and CNT channel restrain condition on driving force and transportation efficiency are investigated. It is found that in order to initiate the transportation, the pretwist angle must be larger than certain threshold, 80 deg, for the case of one water molecule in a restrained (8,0) CNT. Furthermore, driving force decreases with increasing water mass and it is more efficient to transport multiple water molecules than one water molecules. The water molecule is found to have higher degrees of collisions in a (8,0) CNT in elevated environmental temperature. By comparing three CNT channel lengths, the channel length of 19.80 nm is identified as a faster and more efficient transporter in an unrestrained (8,8) CNT. Finally, molecular dynamics (MD) simulation indicates that a water molecule can only be transported below 300 K in an unrestrained (8,8) CNT due to the large friction caused by severely deformed channel and the Brownian motion.