Treasury Secretary Donald T. Regan said yesterday that he is willing to consider changes in his tax reform proposal -- including amending its depreciation schedule -- if enough industries show that they and the economy would be hurt by parts of his plan.

Regan, in an interview yesterday, said that business groups had complained to him about the elimination of the 1981 accelerated depreciation schedule and the investment tax credit, claiming that it would hurt their international competitiveness and increase their costs of capital.

The groups, however, had not finished calculations of what their tax bills would be under the tax reform proposal, Regan said. He said he urged the businesses to do so before they complained about his plan.

Regan was asked whether he would return to the 1981 accelerated depreciation plan, which was the centerpiece of President Reagan's 1981 tax cuts, if there were enough complaints about its repeal.

"If we see that we're irreparably harming any particular industry or segment of the economy -- individual or corporate -- sure, we'll talk about a change in that," Regan said.

"We're not going to hold to a premise for stupid reasons. But on the other hand, if it's just that some individuals are being hurt or individual corporations are being hurt by just having to pay a few more taxes, that's something else again."

Regan met with the National Association of Manufacturers yesterday, which he said complained that companies it represents would be hurt by his plan.

Regan also said that he believes he has successfully beat back opposition to a revenue-neutral tax reform plan.

Some leading congressmen have said that they opposed spending a lot of time on a tax reform proposal that wouldn't raise revenue to help reduce the federal deficit.

Regan said that the budget cuts the president will send to Congress should allay any calls for turning a revenue-neutral tax reform proposal into a tax increase to reduce the deficit.

Regan also said that he was concerned about the course of the economic recovery, saying that growth in the fourth quarter this year will not be "as robust as we had anticipated earlier in the year."

He added that the growth estimate of 6.5 percent for this year over last year probably will have to be lowered.

Regan said he was less optimistic about the economy than he was about two months ago. "That's why I spoke out on the money supply at the end of October. I was worried about this.

"Can we recover?" Regan asked. "Well, a lot will depend on confidence, a lot will depend on how the money supply is worked out and a lot will depend on the level of interest rates."

Regan was asked whether he would change his forecast for next year of 4 percent growth.

"I don't think so at this point," he replied. "Not until we see more of the figures" for fourth-quarter gross national product.

He said a 4 percent growth figure may slip down to 3.8 percent, "but it isn't going to do that much damage."

The Reagan administration is counting on a high growth rate figure in an effort to make the deficit appear smaller.

"I suspect the fourth quarter is going to look a lot like the third quarter," Regan said.

Economic growth for the third quarter was 1.9 percent, down from 7.1 percent in the second quarter.

Regan said that merchants are having a hard time pushing their goods and that it appears sales are "good, but not great."

He reiterated his position that budget deficit reductions should take precedence over tax reform, but that an administration tax simplification plan probably will be sent to Congress in mid-February, following the budget.

Asked whether it was possible that the president would scrap the whole tax simplification idea, Regan said it was hard for him to say because he is so wrapped up in it. "I can't see anyone not saying it's the best thing since cream cheese."

Regan said he can see some changes "made here or there." But if the president "sends it up we're going to fight like hell to get it."

In a related development, former president Gerald Ford said yesterday that he supported Regan's tax reform proposal, but that tax simplification in itself would not raise significant revenue and could not be put into effect before late 1985 at the earliest.