Different polyaniline (PANI) micro/nanostructures, such as hollow microdisks, nanospheres, and nanofibers, have been synthesized in the presence of polyvinylpyrrolidone (PVP) by changing the pH values of reaction system through chemical oxidation polymerization. The formation mechanism experiments indicated that the disk-shaped PANI originated from cycloidal nanoplates, which then self-assembled into disk-shaped PANI. And pH value has an important effect on the arranged state of PVP chains and further impacts the micro/nanostructures of obtained products. It is found that the obtained PANI micro/nanostructures exhibit better electrochemical behavior in HCl solution with the decrease of pH value of reaction system, indicating a potential application in biosensor.