解決辦法：

在列的繫結屬性里加上UpdateSourceTrigger，示例XAML如下

 <DataGrid Name="dgProducts"     IsReadOnly="False" CanUserAddRows="False"   Grid.Row="1"
                   CanUserDeleteRows="False" AutoGenerateColumns="False" Hyperlink.Click="dgProducts_Click">
            <DataGrid.Columns>
                <DataGridTextColumn Header="產品編碼" IsReadOnly="True"  Binding="{Binding Path=ProductNO, Mode=TwoWay}"/>
                <DataGridTextColumn Header="產品名稱"  IsReadOnly="True" Width="150" Binding="{Binding Path=ProductName, Mode=TwoWay}"/>
                <DataGridTextColumn Header="規格型號" IsReadOnly="True" Width="150" Binding="{Binding Path=ProductSpec, Mode=TwoWay}"/>
                <DataGridTextColumn Header="生產數量" IsReadOnly="False"   Binding="{Binding Path=MakeNums, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
                <DataGridHyperlinkColumn Header="操作"  Width="*"   Binding="{Binding Path=PCmd}" />
            </DataGrid.Columns>

        </DataGrid>