WASHINGTON—The Federal Reserve resorted to a series of shock-and-awe stimulus campaigns to stabilize the economy after the financial crisis. Now the Fed is preparing the final move to unwind its support—and it wants to be as boring as possible.

The central bank is likely to announce Wednesday it will start slowly shrinking its $4.2 trillion portfolio of mortgage and Treasury bonds purchased during and after the financial crisis. It will do so passively by allowing some bonds to mature without replacing them next month.

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