1.看很多资料还是有问题 于是想到可以用转换器来实现
这是原来的方式

    @NumberFormat(value = "#.##%")
    private BigDecimal proportion;

请看大屏幕

import com.alibaba.excel.converters.Converter;
import com.alibaba.excel.enums.CellDataTypeEnum;
import com.alibaba.excel.metadata.GlobalConfiguration;
import com.alibaba.excel.metadata.data.ReadCellData;
import com.alibaba.excel.metadata.data.WriteCellData;
import com.alibaba.excel.metadata.property.ExcelContentProperty;
import com.alibaba.excel.util.NumberUtils;

import java.math.BigDecimal;
import java.text.ParseException;
public class BigDecimalAddPercentConverter implements Converter<BigDecimal> {
    public BigDecimalAddPercentConverter() {
    }

    public Class<BigDecimal> supportJavaTypeKey() {
        return BigDecimal.class;
    }

    public CellDataTypeEnum supportExcelTypeKey() {
        return CellDataTypeEnum.STRING;
    }

    public BigDecimal convertToJavaData(ReadCellData<?> cellData, ExcelContentProperty contentProperty, GlobalConfiguration globalConfiguration) throws ParseException {
        return NumberUtils.parseBigDecimal(cellData.getStringValue(), contentProperty);
    }

    public WriteCellData<?> convertToExcelData(BigDecimal value, ExcelContentProperty contentProperty, GlobalConfiguration globalConfiguration) {
        return new WriteCellData<>(value+"%");
    }
}

其实也很简单哈
修改后的

	@ExcelProperty(converter = BigDecimalAddPercentConverter.class)
    private BigDecimal proportion;