SOUTH KOREAN PRESIDENT Park Chung Hee's decision to release 106 political prisoners is no less welcome for its having an evident political purpose. The dissidents to be freed include the prisoner best known abroad, Kim Dae Jung, who took 45 percent of the presidential vote in 1971 and subsequently was jailed for denouncing President Park's dictatorial ways. The other Korean prisoner well known outside his country, Kim Chi Ha, who had won honor both as a poet and as a dissident, had his life sentence commuted to a 20-year prison term. They and the others are affected by decrees issued for President Park's inauguration today for a new six-year term. He has been president since seizing power in a coup in 1961.

We underline the overseas reputations of two prisoners because of the strong suggestion that President Park decided on an amnesty to soften the Carter administration's criticism of his human-rights record. In November, Mr. Carter in effect offered to confer on President Park the prestige of a summit meeting in 1979 if Mr. Park would do something meaningful on human rights. The traumatic "Koreagate" scandal, with its perceived threat of American abandonment, was seemingly behind them. But the even more traumatic American troop-withdrawal plan, carrying to Koreans the same threat, still stretched ahead. Mr. Park presumably thought it was a good moment to cut some of his losses. Jimmy Carter, dangling the summit, made it worth his while.

There is, we grant, something wrong about giving President Park credit for freeing people whom, by American lights, he should not have locked up-especially when perhaps 200 other political prisoners remain in jail and the political system allowing a president to lock them up remains unchanged. But it is worth pointing out that President Park has his own domestic reasons to appear conciliatory. He does not rule as a total dictator. In recent parliamentary elections, his party was actually outpolled for the first time by the leading opposition party, 34 percent to 32 percent: Fewer than one-third of the voters endorsed his leadership. His dominance in parliament is ensured only by a constitutional provision (he wrote the constitution) allowing the president to appoint a full third of the members.

One wonders whether the standing in the United States that President Park has lost over the years by the way he treats his non-communist political rivals has been justified by the political stability and control he supposedly has gained. Or whether he thinks so. Regardless, we suspect now that, whatever the threat Mr. Park may feel from his dissidents, a deeper challenge arises from the Korean underclass. Most South Koreans seem to be as fiercely anti-communist as the president; that is the irony and the waste of his persecution of dissidents. The country's explosive economic growth, however, has produced a class of citizens whose expectations for a better life have been aroused even as they have come to feel they are not getting their proper share of the fruits of growth. Not in looking for communist agents among the elite but in dealing with the legitimate grievances of the peoples does the Seoul government's challenge now lie.