The Treasury's tax-simplification plan would cut private charitable contributions in this country from a projected $59.5 billion in 1985 under current law to $47.7 billion, according to a study by Duke University Prof. Charles T. Clotfelter.

The study was released by Independent Sector, an organization of about 600 charitable groups and foundations that opposes the plan's proposed abolition of various tax deductions for charitable donations. The group argues that this would reduce contributions.

Independent Sector also released a study by Harvard University economist Laurence B. Lindsey, which estimates that without the Treasury plan, charitable contributions would reach $82.1 billion in 1989, but with the plan would be only $69.1 billion, even if the economy grew at an accelerated pace because of stimulation from the tax-simplification proposals.

Lindsey said that if the plan did not lead to accelerated economic growth, charitable collections in 1989 would be $64.8 billion.

According to Clotfelter, the Treasury proposal would lessen projected 1985 contributions to religious organizations from $43.5 billion to $35.8 billion. Donations to higher education would drop from $2.9 billion to $2.1 billion, he said. Combined appeals would go from $3.3 billion to $2.5 billion, and contributions to medical institutions would do the same, he said.

The tax-simplification plan would eliminate several charitable deductions as part of an overhaul of the tax system. Among the changes: the so-called "above-the-line" deduction for charitable contributions by people who do not itemize would be abolished; for those who do itemize, only contributions in excess of 2 percent of income would be deductible. Contributions of property would be valued for tax-deduction purposes at their acquisition price plus adjustment for inflation, rather than current market value.