Mathf.Atan2 反正切2

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
	public Transform target;
	void Update() {
		Vector3 relative = transform.InverseTransformPoint(target.position);
		float angle = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
		transform.Rotate(0, angle, 0);
	}
}

// Usually you use transform.LookAt for this.
//通常使用transform.lookAt.
// But this can give you more control over the angle
//但这可以给你更多的对角度的控制

var target : Transform;

function Update () {
	var relative : Vector3 = transform.InverseTransformPoint(target.position);
	var angle : float = Mathf.Atan2(relative.x, relative.z) * Mathf.Rad2Deg;
	transform.Rotate (0, angle, 0);
}