Kim has changed the officials filling 97 out of 218 military, party and government posts since he took over the communist dynasty in December 2011, the South's Unification Ministry said in a report.
He has often used a "demotion and reinstatement" process in reshuffling military posts in an attempt to tighten control over the military, the ministry said.
"This means Kim has completed the dynastic succession of power successfully and faster than expected," Yang Moo-Jin, a professor at Seoul's University of North Korean Studies, told AFP.
The South's spy agency said in a separate report to parliament that Kim had stepped up a campaign to build up a personality cult around himself.
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Despite chronic food shortages, Kim has spent about 300 million dollars on building leisure and sports facilities including a ski resort, the National Intelligence Service said.
The Masik ski resort has been under construction as one of Kim's pet projects.
Kim wanted the facility -- with 110 kilometres (70 miles) of multi-level ski runs, a hotel, heliport and cable cars -- completed by the end of the year.
Jong-Un, grandson of founding leader Kim Il-Sung, took over when his own father Kim Jong-Il died of a heart attack in December 2011.