通过另外一种方式实现Player的移动控制：

通过EventSystems实现屏幕上的手指拖拽，实时记录手指移动的距离；
通过MoveTowards 参照手指移动的距离，移动Player;
通过Mathf.Clamp限制屏幕边界，使得Player不会超出屏幕边界；

EventSystems实现

新建ugui的panel，设置全屏；
新建PlayerTouch脚本，拖拽到panel上；

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class PlayerTouch :MonoBehaviour,IPointerUpHandler,IPointerDownHandler,IDragHandler
{
    //单例
    public static PlayerTouch instance;
    //拖拽距离
    public Vector2 dragDistance;
    //是否触摸屏幕（按下屏幕）
    public bool isTouchDown = false;
    //按下位置
    Vector2 downPosition;

    void Start ()
    {
        instance = this;
    }

    //触摸离开事件（手指或鼠标离开事件）
    public void OnPointerUp (PointerEventData eventData)
    {
        //Debug.Log ("OnPointerUp");
        isTouchDown = false;
        PlayerMove.Instance.resetInitPosition ();
        dragDistance = Vector2.zero;
    }

    //触摸按下事件（手指或鼠标按下事件）
    public void OnPointerDown (PointerEventData eventData)
    {
        //Debug.Log ("OnPointerDown");
        downPosition = Camera.main.ScreenToWorldPoint (eventData.position);
        isTouchDown = true;
    }

    //拖拽事件
    public void OnDrag (PointerEventData eventData)
    {
        if (isTouchDown) {
            //Debug.Log ("OnDrag:  " + eventData.position);
            Vector3 position = Camera.main.ScreenToWorldPoint (eventData.position);
            dragDistance = new Vector2 (position.x, position.y) - downPosition;
        }
    }
}

PlayerMove实现

using System.Collections;
using System.Collections.Generic;
using UnityEngine;


public class PlayerMove : MonoBehaviour {
    public static PlayerMove Instance;

    //速度
    public float speed = 5f;
    //刚体
    private Rigidbody2D rigidbody;
    //0速度
    private float velocity_zero = 0;
    //初始位置
    private Vector2 position_init = Vector2.zero;

    void Awake ()
    {
        //获得刚体并设置速度
        rigidbody = GetComponent<Rigidbody2D> ();
        //rigidbody.velocity = new Vector2 (velocity_zero, velocity_zero);
    }

    public void resetInitPosition(){
        position_init.x = transform.position.x;
        position_init.y = transform.position.y;

        Debug.Log ("position_init:  "+position_init);
    }

    void Start(){
        Instance = this;
        resetInitPosition ();
    }

    bool isTouch = false;
    float h;
    float v;

    void Update ()
    {
        onTouchScreen ();
        move ();
    }

    private void move(){
        //判断是否触点了屏幕
        if(!PlayerTouch.instance.isTouchDown){
            return;
        }
        //获得目标位置
        Vector2 target =  position_init + PlayerTouch.instance.dragDistance;
        //移动到目标位置
        transform.position = Vector2.MoveTowards (transform.position, target, 0.1f * speed);
        //限制移动范围
        transform.position = new Vector2 (
            Mathf.Clamp(transform.position.x, AppCommon.getLeft(), AppCommon.getRight()),
            Mathf.Clamp(transform.position.y, AppCommon.getBottom(), AppCommon.getTop()));
    }

    /// <summary>
    /// 根据触屏事件，修改app运动参数
    /// </summary>
    private void onTouchScreen(){
        if (Input.touchCount <= 0 && !Input.GetMouseButton(0)) {
            AppCommon.app_velocity = 0.05f;
        } else {
            AppCommon.app_velocity = 1f;
        }
    }
}