使用java.awt.Graphics2D的setStroke方法就可以解决这个小问题

一个Graphics对象可以强制转化为Graphics2D类型(向下转型)

Graphics2D g2 = (Graphics2D)g;  //g是Graphics对象
g2.setStroke(new BasicStroke(3.0f))

public class MyPanel extends JPanel{
		protected void paintComponent(Graphics g) {
			super.paintComponent(g);
			Graphics2D g2 = (Graphics2D)g;  //g是Graphics对象
			g2.setStroke(new BasicStroke(3.0f));
			
			g2.setColor(Color.GREEN);
			
			g2.drawLine(0, 0, 500, 600);
		}
	}